Philip, please read my post again, then compare with OP. It is the same problem.
You can test on your default install.
You write:
We have investigated the subject, and noted significant variations from one email service to another, which is classic, but in fact Gmail app diverges much more and we are therefore going to work on this subject.
Note that only with Gmail app (so far) regarding your specific problem.
In gmail and with price format 00,xx €, you have problem. So all EU with gmail customers. 30% of customers? Not so specific.