If you tried both drivers in both axes, and the problem still appeared on the DEC axis only, then it cannot be the drivers.
It may be some soldering issue or motor wiring issues (all of that being unlikely, but can't think of anything else).
The Blue Pill board is good. I used it for a few years.
The only issue is that the STM32 module is slow.
There is a better module that is pin compatible, which is the RobotDyn STM32F303CC.
It is faster and has more flash storage.
But it was hard to get last year. No idea if RobotDyn has inventory now.
There is also the MiniPCB, and the S6.
The former allows MICROSTEPS_GOTO, but the latter only allows it only for SPI drivers (TMC2130 and TMC5160).
Each has pros and cons.