Damn... my only guess: time machine backups are VERY write heavy. Is it a reputable brand of SSD? Is time machine frequently erasing old backups? Many external portable SSDs that work fine for regular consumer uses, might struggle to handle the write volume of time machine backups. I've used 2 external SSDs with time machine and never had them fail. (T7 Samsung SSDs)

Also you'd want the SSDs to be much larger than what you need to backup. If they're small, then time machine will have to erase the older backups to make new backups, which just leads to more rewrites, stressing the SSD more. The ones I used were 4TB, but a clean first backup of my system was only like 250GB (because many things excluded). And then daily incremental backups are like 1-3GB.