> At this point, I kinda doubt this issue has anything to do with Debian > itself, but will most likely be an issue/limitation of the Linux Kernel > itself.
>From my latest tests, it seems to point that way. Kernel 5.4 came with a new mount API and it seems to break since then. During my search, i also found that since kernel 4.19, there is a default limit of mount set to 100 000 to avoid DOS. We're still far from it.
