Everyting put on default is deleted on reboot of the service.
Oh yeah, using .../default/mibs does NOT work.
But to be clear, on an RB1100 at least, putting them in <dude_data_directory>/files has always works to find mibs, without issue in my experience. If <dude_data_directory> is a disk (and it should be), the MIBs are persisted after reboot.
I believe your right that <dude_data_directory>/files/mibs should work – but I recall that NOT working sometime in some versions & why I started just using "files" under the Dude data directory.
It may also help that the mibs have a .txt file extension, I've also made sure they end in .txt not .mib. I recall some issue with the file types at some point too.