Omg, that's it!
I sear I've tried to enter a near-zero value before and it didn't save. But it does seem to save if the fractional part has no more than 8 digits. *self-facepalm*
It works wonderfully now (both in the Cointracking tables and the tax reports) without any workarounds I might forget it a year. 


Thanks a bunch for all the help, much appreciated! ♡