I recall having something vaguely similar with another model where it was resolved once I upgraded the firmware (RouterBOOT) to the version included with the same RouterOS and restarted it. It may be worth a shot if a newer one is available.
That is the first thing i did, bit it didn’t help. I have downgraded to 5.22 and firmware to 2.39, and everything is working ok for now. I have sent supout.rif to mtk, and maybe they will look at it and give an solution.