Thanks for the reply Jen_D

We are using the QuickBooks App (Mac)
Our accountants are using QuickBooks App (PC)
I've also just checked the same process on Chrome (Mac).
Don't think it's a caching or cached currency conversion.
From my screenshots you can see that the currency conversion is consistent in both the initial statement transaction calculation, and when moved to the bank register. 

The mention of reconciling is irrelevant now, as the figure in the bank register doesn't match the actual statement.

Will take your advice and see if there is a way of escalating to the QB support.