Did you test some other client device behind the switch pulling a lease? How that differs from the switch as a client?

Bizarre. Hard to see what might be different there.
One possible test you could do would,be to assign mvneta0 as the LAN on the 2100 to remove the on-board switch. That's quite involved though.

@brepo

I feel a little sorry for myself, because I spent more than 10 years with pfsense and everything suited me before :)