Had a few of these encoders start becoming inconsistent and finally got around to fixing them today. Didn't bother identifying the parts but they didn't exactly match any of the Bourns/Alps/Panasonic encoders I'm familiar with.
Desoldered from board, pried open the encoder assembly and cleaned in flux cleaner + lubricated the contacts with MG chemicals 801B, then pressed the assembly stack back together and reinstalled.
There was the usual build up of dust etc which makes encoders stop responding, jumpy, or inconsistent (be they electrical or optical types). All back to smooth and predictable controls now