Is this an Atheros card? I'd similar problem which comes down to the data in the ART EEPROM. Conversation starts here: 5GHz Low performance on TP-Link Archer C6 V2 - #92 by Cthulhu88
There is the antenna gain, which the driver takes into account, but it's very odd that it has different (capped) max power between regions where the rules are the same. I assume it's a bug with the OpenWrt patches that modify power based on antenna gain, which is probably reading incorrect data from the EEPROM.