Inconsistent Timebase

With my DSO203 I have a nasty issue with frequency measurements.
When feeding a 500kHz signal into the device I am getting different period measurements and frequencies when I change timebase:
5us: 500 kHz ok
2us : 520 kHz
1us: 625 kHz
0,5us: 833 kHz
0,2us: 1042 kHz

My software is SYS_B152.HEX + APP_B253.HEX and V261FPGA.BIN
Is this a known issue or is this a bug of my device?
What can I do to get consistent measurements?
Thanks in advance!

Update: Revertion to older Firmware incl FPGA-Version and helped. Things are working now with
SYS_B151 + APP_252 and V261FPGA.BIN. Still inconsistent timings with gabanator’s firmware.

Following observation:
In the firmware sources BIOS.C got from Seed-Studio’s github
( … .50/source )

there is a structure X_ATTR for the timing.
Values in the column KP are 1024 until 2us/DIV where the values are getting smaller.

Feeding 500 kHz from internal signal generator and taking measurements and comparing them with the KP values:
5us/DIV KP=1024 Meas f=500 kHz
2us/DIV KP= 983 Meas f=520 kHz
1us/DIV KP= 819 Meas f=625 kHz
0,5us/DIV KP=614 Meas f=833 kHz
0,2us/DIV KP=492 Meas f=1042 kHz

Following phenomenological coincidence:
KP[ i ] / KP[ j ] = f[j] / f[i]
This is not random. I don’t know exactly what KP is for but for correct visualization and measurement it should be considered.

I can’t imagine that I am the only one having this problem with the wrong measurements.
Could someone reproduce this?