There seems to be some sort of conflict with PolyMC and Moonlightlib. The server starts, but if I try to connect from a client that doesn't have moonlightlib, I get this monstrosity of an error message. (it does not show up in the log so a screenshot is the only way I can capture it)

If I add a server mod that requires moonlight I get an even bigger monstrosity.

If I add moonlight to the client, I can join.

if I add a mod that requires moonlight to the server, but not the client, I get a much smaller error:

Internal Exception: io.netty.handler.codec.OecoderException:
Java.lang.NullPointerException: No map decoration type with id:
supplementaries:banner[]

If I add a mod that does not require moonlight to the server, it works as expected.

Tested with:

MC: 1.20.1
Fabric: 0.14.22
Fabric API: 0.89.0
PolyMC: 5.4.0
Moonlight: 2.8.28
Supplementaries: 2.6.2