overflowgid & overflowuid

file: /proc/sys/fs/overflowgid & overflowuid
variable: fs.overflowgid & overflowuid
Official reference

Some filesystems only support 16-bit UIDs and GIDs, although in Linux UIDs and GIDs are 32 bits. When one of these filesystems is mounted with writes enabled, any UID or GID that would exceed 65535 is translated to a fixed value before being written to disk.

These sysctls allow you to change the value of the fixed UID and GID. The default is 65534.