I realize this is VISA/MC, but I think I'm having a similar issue with Discover.

Yesterday I succeeded in using my Discover It card in Japan at UNIQLO by telling the cashier it was a JCB. And currently I wish I hadn't succeeded. The charge is pending on my Discover account with merchant name "JCB MERCHANT" in a USD value which when converted from my JPY receipt means the exchange rate used at transaction time is 1 USD = 106.97 JPY. The actual exchange rate for yesterday is 1 USD = ~118 JPY.

I'll report back when the charge posts, but so far I wish I had used my VISA or MC because none of the merchants I've used those at have applied DCC.

update: the transaction posted with an exchange rate of ~118.30. Looks like I'll keep using Discover in Japan then!