iPod 5th Gen/ iPod nano not able to sync whole music library

I'm trying to sync both an iPod 5th Gen (80Gb, unmodded) and iPod nano 1st Gen (2Gb, unmodded) to my M4 Mac mini (macOS 15.6.1).

For some reason Finder only provides access to some of the recently added items from my library which has >45k songs, >5300 albums. (I've not checked the sync button until it gives me the whole library as it'll wipe what's already on the iPod). Is there any way to force Finder to use the whole library? It used to do this no problem.

[Edited by Moderator]