The XML Source help page lacks information that this is an experimental feature. I would test a file in a bug report but the Data > 'XML Source' menu item in Calc was deactivated (greyed out). In the web I found the info that this is an experimental feature and I had to activate that in the options. Other users, maybe one who don't know about experimental features, will hardly find this info or will know anything about experimental features.
Add an info box that this is an experimental feature and a link to https://help.libreoffice.org/6.1/en-US/text/shared/optionen/java.html?DbPAR=SHARED#bm_id4077578 -- maybe with a warning that experimental features are not for average users due to crashes, the end of the world, whatever.
The note can be added with
Ilmari Lauhakangas committed a patch related to this issue.
It has been pushed to "master":
tdf#120221 Add experimental note