[b37 + b38] Vanishing Portal to Angolwen

[Feldherren]

Each time, without fail, on a non-Archmage character I complete the apprentice mage's quest and gain access to Angolwen. The portal is perfectly usable, and I've always gone in.
However, after leaving to go do something else - complete Old Forest, or anything, really - and returning to the world map, the portal tends to disappear.
This has happened for me in both b37 and b38. A similar thing happens with the Graveyard; if I don't attempt it immediately, it vanishes and I lose the chance, though I haven't yet tested this in b38.

I tend not to visit Zigur, and don't have anti-magic on my current character, so whilst I'm told that will make the portal disappear, that isn't it in this case.