If I open a .docx file with version 6.2.n of Libreoffice and save it as .odt then I cannot reopen the file with Libreofffice 6.2 though version 6.1.5 will open it.
If I open and save the ,docx file with version 6.1.5 then both 6.2.n and 6.1.5 will open the resulting .odt file
I am running 64 bit Ubuntu 18.04
I cannot claim it’s every file but the problem has bitten me twice. The first time I did not investigate and gave up.
To do something with this problem, it’s required that a proper bug request be filed, with a sample docx attached, which would produce broken ODT on save with 6.2.x. Without that, it’s nothing that can be done, the more so as you say “I cannot claim it’s every file” - of course that’s expected, as otherwise it would be caught early, and already fixed.