Chain slipping off bottom jockey wheel

[Fifelad]

Relatively new jockey wheels, new chain, cassette.
Started to doubt myself if I had the jockey wheels the wrong way around, but it’s been ok for ages. Only other thing I’m thinking is they were BBB with adaptor rather than Shimano like for like replacement and it’s a minuscule bit out of kilter. Ideas please.

 

[Ajax Bay]

Check your hanger alignment. The chain is vertical (line drawn between the tension run at the top and the return run at the bottom). If the cage plates are vertical (as they should be), then the jockey wheels axes are perpendicular to that and the wheels themselves run in plane parallel to the plane of the bike.
Unless your chainline is way out and this happens in small/small (ie cross-chaining) perhaps. Is the chainset newish? Was the BB changed?