Cytron Maker drive running at different speeds

Hi all, hope you're doing ok wherever you are in the world.

I've recently been using a Cytron Maker Drive, with 2 dc motor control. It's using PWM_PWM, and after having setup the 2 motors in code, I set the speed to 128 (max is 255, min is -255) to make both motors go at 50% speed for x amount of time. However, both motors do not go at en equal speed, the left goes considerably faster that the right, and if I switch the connectors for the L/R motors, the right one goes a lot faster.
By the way, the drive I'm using is this one.

Any ideas for troubleshooting?

It is fully normal that 2 different motors run differently fast at the same powering.
You ned to calibrate them. Let one motor run at 128 PWM and check other PWMs for the other motor until they match better. Totally the same speed is impossible without some kind of RPM feedback and controlling code.