Had this issue before, can confirm.

From what I understand, this is the ghost unit issue, and it occurs somewhat randomly. It is not specific to beasts; it may occur with any unit.

It actually makes sense to me the way it is: lava acts the same as liquid gold, hence the sentinel should be immune to it. It also gives the player a unit which can traverse lava/liquid gold, it would have a lot less use in the hands of the Empire AI.

I think this is intended.

I believe that it is the ghost unit issue from here (the second type):

https://brightrockgames.userecho.com/forums/1-war-for-the-overworld/topics/2730-ghost-units-in-skirmish/

It would be. Any map which features obsidian is going to have the glitch. The more obsidian there is, the more prominent it is.

I have noticed two of the issues you mentioned as well. The obsidian texture glitch is something which happens in any game mode. Not only does the side texture disappear but it seems to randomly appear elsewhere, leading to floating walls. The 'imp' typographical error is somewhat prevalent, here is one I encountered recently:

'Your "Minion" has been converted in an enemy torture chamber' I believe. It came from the cultist.

This is still at large.

Seems to be the cultist description.

Output log:
output_log.txt

Very possible, I may have been using the console at the time.