I've not found a solution - other than to drop the CBUUID filter completely and sift through the noise. I've filed 'feedback' against the current 18.1 beta - but I fear this is intentional.

What appears to be broken is decoding the 'Overflow' area (/manufacturer data) - which is where the missing services are hashed down to a bitfield when he peripheral is backgrounded. Without the service filter - with the iOS 18 central in foreground, you do get advertisements from the backgrounded peripherals, but they have no listed services - which is true of the actual advertisements being broadcast - and the manufacturer data has been stripped (which is needed to put the services back).

Thanks for posting - I'm just updating to 18.1 beta 7 now (it wasn't fixed in 18.1 beta 6). Fingers crossed I can retire my awful workaround soon!