Well it is a constant, it is the default value of EntityPersistenceConfig.Priority, so if a vanilla config (or modded config I guess) does not define a specific priority that is the one it gets, and since EntityPersistenceConfig is compiled that is not apparent from the code and only becomes apparent via workbench or trial and error

For anyone searching this issue, the magic number is 32500, thats the minimum priority needed to override a vanilla config. This should probably be documented and/or mentioned in code comments because the default priority isnt accessible anywhere, I had to discover this through trial and error

Still occuring in 1.8.0.10, or did you mean a future 1.8.x version?

Your snippet passes an explicit single-id Include, which works reliably for us too. Our failing requests set Collection with no Include list at all.

I'll capture a -loglevel spam -persistence dig log of both modes on 1.8.0.10 and attach it as soon as I get some time to look into it

Yes, there was existing save data, however the entity being tested (Game Master spawn of E_ArsenalBox_US) was freshly spawned with the override config registered before the world loaded, and its record still landed under vanilla's Arsenal.conf collection/serializers in every variant (Priority 30000 and Priority 1 behaved identically, which looks like the mod config was never in the competition at all rather than losing it).

I will setup another test with no save data to see if it helps