I have already raised this in another thread (Hidden commission on currency exchange?) and I had a chat with the support the other day. They acknowledged it, but told me this was not a bug and this is how it’s supposed to work and it’s because of market ‘fluctuations’. When I argued that if it was due to fluctuations it would sometimes be in my favour, whereas what I see is it’s consistently not the case, they could not tell me anything.
Bottom line is, there is now a hidden commission, at least on GBP/EUR pair, and Revolut’s claim that they offer interbank rates without any markup is sadly no longer true 
The markup is really small and the rate is still competitive. It’s just not interbank anymore. What’s really disappointing is that they introduced it sneakingly without letting anyone know. But, hey, Revolut has a long history of PR triumphs, part of their DNA 