The pictured amulet doesn't improve my stealth mastery at all.
I am a cornac rogue, and I have placed an extra category point into stealth to increase it from 1.30 to 1.50. When I wear the amulet, my mastery stays at 1.50, and none of my stealth-related abilities improve.
When I wear the Skin of the Many, my stealth mastery is reduced to 1.30. Even then, wearing the amulet does not improve my stealth mastery.
I've made a copy of my save file in case you should need it.
[b34] Broken stealth mastery amulet
Moderator: Moderator
Re: [b34] Broken stealth mastery amulet
You can spend category points into learned categories for a bonus? Ffffffffffff.
Re: [b34] Broken stealth mastery amulet
You made me laugh like a fool, Rect. 
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The gear below doesn't appear to affect my stealth rating, either.
My stealth efficiency is 70.
Equipping the cloak doesn't change that value on my char sheet, my level up page, or the stealth icon tooltip.
Something seems broken with stealth gear. However, when last I checked, artifacts that affect stealth (Skin of Many, Serpentine Cloak) seem to work properly.

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The gear below doesn't appear to affect my stealth rating, either.
My stealth efficiency is 70.
Equipping the cloak doesn't change that value on my char sheet, my level up page, or the stealth icon tooltip.
Something seems broken with stealth gear. However, when last I checked, artifacts that affect stealth (Skin of Many, Serpentine Cloak) seem to work properly.
Re: [b34] Broken stealth mastery amulet
Grimwerk, I think this all can be explained. Since Stealth is only checked when activating it, not when switching equipment, the effects aren't added until you re-activate Stealth. Mastery changes the talent directly, so it's effect gets accounted for when you re-activate Stealth. I don't think there is anything that checks stealth bonuses from equipment, and the player's stealth level gets overwritten when you activate Stealth, so it has no effect. Those items should probably be changed to give small bonuses to stealth mastery, or the Stealth talent needs to be rewritten to account for this equipment.
Actually, what this doesn't explain is why the stealth mastery amulet didn't change the talent at all. I wasn't able to reproduce that. I didn't use a stealth mastery amulet, though, so maybe it's a problem specific to that tree.
Actually, what this doesn't explain is why the stealth mastery amulet didn't change the talent at all. I wasn't able to reproduce that. I didn't use a stealth mastery amulet, though, so maybe it's a problem specific to that tree.
Sorry about all the parentheses (sometimes I like to clarify things).
Re: [b34] Broken stealth mastery amulet
I'm lost, Bricks. 
While stealthed, I can reduce my cunning from 88 -> 80 (by removing a sneakthief ring).
The presently active stealth value on my character sheet remains the same at 70.
The on activation stealth value on my stealth icon tooltip lowers to 64.
If I reactivate stealth, my character sheet value drops to 64, as expected.
Wearing or removing the shadow cashmere cloak of fog makes no change to any apparent stealth value when I activate, deactivate, or reactivate stealth. Restarting ToME doesn't help, either.
Wearing or removing the amulet makes no change to any apparent stealth value when I activate, deactivate, or reactivate stealth. It makes no change to my stealth mastery level on my Talents Levelup page, either.
Wearing the Skin of Many reduces my stealth mastery level on the Talents page from 1.50 to 1.30.
The presently active stealth value on my character sheet remains the same at 70.
The on activation stealth value on my stealth icon tooltip lowers to 57.
If I reactivate stealth, my character sheet value drops to 57, as expected.
I think all this confirms what you've said.
I've added it just to be clear.

While stealthed, I can reduce my cunning from 88 -> 80 (by removing a sneakthief ring).
The presently active stealth value on my character sheet remains the same at 70.
The on activation stealth value on my stealth icon tooltip lowers to 64.
If I reactivate stealth, my character sheet value drops to 64, as expected.
Wearing or removing the shadow cashmere cloak of fog makes no change to any apparent stealth value when I activate, deactivate, or reactivate stealth. Restarting ToME doesn't help, either.
Wearing or removing the amulet makes no change to any apparent stealth value when I activate, deactivate, or reactivate stealth. It makes no change to my stealth mastery level on my Talents Levelup page, either.
Wearing the Skin of Many reduces my stealth mastery level on the Talents page from 1.50 to 1.30.
The presently active stealth value on my character sheet remains the same at 70.
The on activation stealth value on my stealth icon tooltip lowers to 57.
If I reactivate stealth, my character sheet value drops to 57, as expected.
I think all this confirms what you've said.
I've added it just to be clear.
Re: [b34] Broken stealth mastery amulet
Yes, and you said it much more clearly than I.
Talent descriptions update dynamically, but sustains effects don't unless they have been designed to; Ruin was, up until recently, in the same position. Further, equipment that increases/decreases stealth level (not mastery) seems to have no effect. Finally, no idea what's wrong with that amulet. It just figures that you find a mastery amulet that you want, and it doesn't work.

Sorry about all the parentheses (sometimes I like to clarify things).