Pulse multiplier board (G901) problems - step loss

Dear all, maybe some of you faced this kind of problem.

I`ve found problem using G210 (=G201+G901) step-motor driver.
In summary - when I use it with pulse multiplier board, step-motor
loose steps, when I use it with pure HCPL2531 optocouple - no problem.

In detail:
Step loss occurs only on low rotation speeds (1 RPM or F100 = feed rate 100), on speeds >5RPM no problem.
Problem can be seen in MACH with simple G-code (continues
forward/backward movement):
1 g1 x 5
2 g1 x 0
3 g1 x 5
4 g1 x 0
....
50 g1 x 0

On step N50 rotor of step-motor have a visible loose of steps.

Problem does not depend much on motortuning parameters (example velocity=3000, accel=200, step/dir pulse 1...5 us). Also problem can be seen with all multiplication coefficients.

Can you comment this problem? Maybe I`m doing smthng wrong with MACH?
Problem is the same with different drivers, I tested 6 drivers
G201/G210 with different multiplying boards, so I can eliminate
hardware fault.

I wrote email to Gecko support, waiting answer from them.