Using dual SIM some data is still used on first data service when set to use second service only

I have a dual SIM iPhone with a UK and a Greek SIMs.  To avoid roaming charges when in Greece I switch off data roaming and switch data to the Greek SIM only when in Greece.  I've checked my bills

and I am still getting a few kB of data being billed each day to the UK service.   It seems the iphone is ignoring the instruction to only use the Greek SIM for data.

I've checked the forums and the problem with data leaking with data roaming switched off is known.  The advice is to switch off mobile data altogether when abroad.   Then you can receive calls but not use data.   However that advice does not work with a dual SIM as it would disable my use of the Greek SIM data.   Why is the UK network still recording roaming data when the iPhone is told to only use the other data service.   BTW, the setting to "auto switch" data services is switched off too.

Seems like a bug in dual SIM software? I don't think this is a carrier problem - it is an Apple problem.