Partially missing guardian

[Arsonius]

There is bear guardian in the ruined dungeon whom I don't see and can't hit with any AOE and who can hit me while I'm in a small 3x3 square. And he doesn't have any form of invisibility or teleport.
Can't upload a screenshot here, it says it's too big.

[Crim, The Red Thunder]

Save it as a jpeg file, insteap of the standard format. (Paint does this, if you're on windows.)

Alternately, shrink/cutoff the unnecessary parts of the image. Or upload it to some image-sharing website and post a link to it here. (The former is better then the latter, though both work.)

[Arsonius]

Character is standing in the center of that 3x3 square where guardian can hit me.

[Crim, The Red Thunder]

Almost sounds like a similar bug that trips on certain DoT effects around an enemy when the enemy dies and effect persists. (Blade horror has done it in the past.) Are you positive that you didn't already kill the enemy in that spot? (Possibly offscreen through hymn of moonlight, or shadow blast, or starfall, or some other AoE attack?)

Edit: Also, there's been a weird bug going around for a few versions where a guardian in the ruined dungeon does not generate appropriately. Not sure if that leaves an AoE effect where they are supposed to be or not.

[Arsonius]

No, I'm sure there was just two normal enemies in the room. At first I thought it is in the walls and started digging around (Had this problem with ghost Tannen sends you to - it just stuck in the same place and don't moved until I digged to it) then noticed that damage is in certain area only. And I right clicked it through the log - it shows 100% HP.
I have even tried using vision rune with detect animals.

[Zonk]

Ran into a similar issue..with the web effect from a spider guardian. Nasty bug, means I can't complete the RD on this char

[0player]

It strangely sounds like the bear is an actor that has its coordinates (so, it can target its abilities, including Attack, properly), but is not on the map (map layer ACTOR does not contain it), so it is not subject to drawing or damage projecting. It's like "reverse clone bug".
This needs to be tested though.

[Arsonius]

It strangely sounds like the bear is an actor that has its coordinates (so, it can target its abilities, including Attack, properly), but is not on the map

That's what I thought as well. Although it doesn't seem like he is using his abilities (except for auras and shield which are sustained) and just keep attacking me. I still have a save if it'll help.