@Frank For the record, that thread (started by yours truly) was about card transactions, and it surfaced that a different, separate bid (i.e. not mid-market) rate applies to them, which the app only shows ex post.
@Grahamnbutler’s situation is that he used the currency conversion in the app, which does use the mid-market rate and shows it in advance (edit: it appears it doesn’t, and in that case the rate shown is not the one used for the conversion). Also, no weekend surcharge applied, since he did it on a Wednesday.
I don’t think there’s any problem here. Assuming the GBP/EUR exchange rate didn’t change much between the two conversions, it would only amount to a commission of 0.2% per transaction, and in reality even this is perhaps mostly due to rounding as @Grahamnbutler was only exchanging the relatively small amount of 5 GBP.