Somewhere between Therion 6.3.1 and 6.3.4 my cave model grew from about 38 km in length to 5,727 km in length.

The shorter distance is more likely to be correct!

I presume it is a Therion bug rather than a bug in my data input.

I have described it a little more in https://github.com/therion/therion/issues/663

Bruce