OK, I did a Composer library update, which updated both DOMPDF and MPDF:
https://github.com/Fabrik/fabrik/commit/91865722be4578a0d855437ecb5f8e0b51727781
... and added a "PDF Bootstrap" option to both form and list settings (same place you set orientation and page size), which now controls whether we include the default Bootstrap CSS files when generating a PDF. Default before was to always include it:
https://github.com/Fabrik/fabrik/commit/df7d0b6a1c7235d46ebf9a2d66f38fb4083e61b9
I don't know whether either the library update or the new option will fix your issue - you can try turning PDF Bootstrap off, although as the tooltip says, rendering a Bootstrap template without the CSS won't be pretty, but if it does work, something is better than nothing.
As for "it worked before 3.9", that's because we updated from a very old version of DOMPDF in 3.9. Which fixed an awful lot of other issues, but has obviously introduced some others. Which is just how life is with PDF libraries, unfortunately. There is no magic bullet, no library that works 100% converting HTML to PDF.
So if you want to do a full github update (see Wiki), and let me know if anything helped ...
Of course, I strongly suggest doing an Akeeba backup before updating, in case the update makes things even worse.
-- hugh