Time Machine fault with backup to SSD drive

I bought a new, unbranded 2Tb SSD drive for backing up my mac with Time Machine. Reformatted the SSD with APFS format. Time machine backup started fine, but did not complete on the first day – then after shut down and restart, Time Machine started the backup from scratch, as though the first (partial) backup had never happened. Tried erasing the SSD and starting again with the same result each time.

Tried reformatting as Mac OS Extended (Journalled), but same problem.

Wondered if this was a Big Sur issue but Time Machine backs up fine to a different (non-SSD) external drive. Also wondered if this is because SSD is unbranded/cheap, but SSD works fine for all other storage uses.

Thanks in advance for any help...

(Late 2014 mac mini, OS Big Sur)