Resolved. Tried this with the latest version of the game (3.6.33.5509 on Jan 12, 2019). Initially I was loading old save files from the Nameless Isle and the issue was still present.
Did several hours of testing identical to the one described above to no avail. Then, even though I was not feeling like continuing to play without the achievement, I still decided to load a later save from Arx. And to my greatest surprise the achievement popped up.
On the one hand, I am grateful and relieved. But on the other, I'm also perplexed why the fix does not apply to save files where the player is on the island itself, let's say after all altars but before the temple.
Anyway, I hope this information helps others who, like me, keep trying only with Nameless Isle saves.
Last edited by Avan; 12/01/19 12:28 PM.
