I had a similar experience a couple of years back buying 4 J seats for the family to travel from Japan to Australia with Qantas.

I booked and paid on the Japanese site in yen using my Australian credit card. The charge initially could be seen to have been applied to my card (available balanced decreased)...but then disappeared and 6 weeks later finally billed at the Exchange rate for THAT time (not time of booking). Difference was 1500 AUD due to a significant FX fluctuation.

Complained to Qantas... they blamed bank/card provider. Complained to bank...they blamed Qantas. Got nowhere with it other than understand-but-nothing-we-can-do except allow you to refund with no charge
response by the local Qantas office.

In the end I left it but was a big learning to avoid big purchases on card in foreign currencies as you can't always control when the charge will hit and with what FX rate.