Fake Memory Card (with crosslinked blocks, bad blocks etc.) or just simply worn out.
"PSU Paste" option taking whole folder contents and pack it into *.psu container which keeps all file attributes, dates and names. This is ultimate save preservation method for PS2 game saves (even better than PSV). "MC Paste" is older solution and it is just copying saves (folder + files inside) + making additional file which contains important meta data for restoring, using the same option. You can still using it if You for some reason prefer, but much more handy is just one file.
However, I don't know which v4.43a are You using (this is wLE, a more modern fork of uLE v4.42e, which is v4.43a in version name but can be different build, recommended version is 2019-01-14). Maybe, just maybe, it have broken PSU Paste/MC Paste which could result in paste failing. I don't know that and I doubt it but in theory it is possible. Try version 4.42e but personally I'm pretty sure that fault is Your mc (even original memory cards are currently ancient and start failing, and one day there will be no more healthy PS2 MC on this planet ;]).