I have several omnitik installed and I have the same problem by about 50%. It happens with the classic model and the AC model.
The problem is that from time to time, all sxt-slaves lose the connection for 3 seconds (with a constant ping there are 3 or 4 timeouts). The sxt doesn't lose register on AP.

I've tried changing the frequencies, but it's no solve the issue. I've tried to go up and down Tx power, but it doesn't work either. The 6.40.9 that optimizes the AP doesn't solve it either. I've had this problem for years. CPU usage is normal, and basic configuration, is a wireless issue

Does anyone know why it happens and how to solve it?

It's a problem that's giving me a headache. Can you think of a reason? Nobody's ever had the same thing?

Isn't it DFS scanning? Or automatic channel selection looking for better frequency?

DFS is off. I try several frecuencys, but with the same result always

I had very similar issues using the OmniTIK PG-5HacD and several SXT G-5HPacD r2 as bridged clients. The SXT radios would connect with very high CCQ. Then slowly the CCQ would degrade and then they would all disconnect and reconnect. I tried changing channels and reducing TX power but it actually made things worse. After many hours over many days I was able to resolve the issue.
Not only did it resolve the issues but the performance is excellent.

OmniTIK PG-5HacD Recommendations:
1- Update to the latest ROS.
2- Increase antenna gain instead of reducing TX power. In my scenario I went from default of 7.5dBi to 9dBi.
3- Be selective with advertised data rates. Example: ht-supported-mcs=mcs-12,mcs-13,mcs-14,mcs-15
4- Use narrower channel widths. Example: channel-width=20/40mhz-Ce
5- Use fewer 802.11 specs. Example: band=5ghz-a/n instead of 5ghz-a/n/ac
6- Use hardware protection mode. Example: hw-protection-mode=rts-cts
7- Use adaptive noise immunity. Example: adaptive-noise-immunity=ap-and-client-mode

SXT G-5HPacD r2 (station-bridge mode)
1- Update to the latest ROS.
2- Increase antenna gain instead of reducing TX power. In my scenario I have 10 SXT radios and I adjusted each one to a different setting. Anywhere from the default of 16dBi to 24dBi.
3- Use narrower channel widths. Match to the AP settings.
4- Use hardware protection mode. Example: hw-protection-mode=rts-cts
5- Use adaptive noise immunity. Example: adaptive-noise-immunity=client-mode

Thank you so much, I'll try!

I had the exact same problem. The problem was the RSTP (rapid spanning tree protocol) in the bridge. Try set to none.

Good luck