Solved: the problem was having the STA Repeater mode turned on while using ethernet WAN. Turning off the repeater mode made the problem go away.
Long story:
I use GL.iNet Beryl AX during travels. Most of the time I use it in hotels, where I extend the wifi from the hall into the room (by using the STA repeater mode). The problem with packet losses started in a rented appartment when I decided to hook up the Beryl through a LAN cable to a router instead.
The logs (dmesg) showed that every 30-40 seconds my GL.iNet Beryl AX was trying to look for a previous hotel's wifi network to connect to. This resulted in me losing my wifi connectivity on the client side for a couple seconds every time.
Turning off STA Repeater mode when Beryl is connected via a lan cable made this problem go away.
Note to developers:
If ethernet is connected, maybe have STA Repeater mode turn off automatically?