Summary:
Ancient mobs not appearing as unlocked after defeat, instead continuing to appear as silhouettes with question marks, achievement "Ancient Hunter" progress not updated.

Expected Behaviour:
Upon defeat of ancient mob, the mob becomes "known" to the player, with the icon being updated and the achievement progress being incremented.
Achievement is expected to update only on the first kill of each respective ancient mob up to 15 unique ancient mob kills.

Actual Behaviour:
Ancient mob does not become "known" to the player (icon remains a silhouette). Achievement progress does not update.

Steps to reproduce:
Begin an ancient hunt at apocalypse+ difficulty, use a combination of runes appropriate to an ancient mob you have yet to defeat. Progress through dungeon and defeat ancient mob. Leave dungeon though exit door. Visit nether portal, and enter offerings menu, add the same runes as previously, the ancient mob will appear as a silhouette (not correctly unlocked), and achievement progress will not have updated.

Notes:
I am playing cross platform with a friend. They are on Xbox One and I am on Windows 10 Pro (19041.804) using Minecraft Dungeons build 1.8.6.0_5665003. This behaviour seems to have been introduced with the latest update as prior to that we were able to progress this achievement, and unlock ancient mobs. We are both experiencing this issue. We have tried playing on numerous difficulties and a variety of different strategies (clearing the whole dungeon, running past things etc). Please don't hesitate to contact me if you need further information.