21. 4. 2021 v 11:41, Tarquin Wilton-Jones via Therion <therion@speleo.sk>:

I have found that stopping the walls on both sides
of the passage in a single scrap will upset kml generation.

Absolutely. I have reproduced the problem on a small scale like this.

This should solve your problem. Anyway, if you will try to imagine such situation as missing wall in passage is it is not real. There should be some  kind off wall to circumscribe passage (emty space) from bedrock.

M.