I give every server a unique group name. Eg: ZNC_EUShell_EFNet_Raccoon

You say that the Group Name is overwritten by the server name asserted in 005. Does mIRC forget about ZNC_EUShell_EFNet_Raccoon when it gets that 005 "EFNet"?

Well. At least I won lunch.
Good philosophy, see good in bad, I like!