I found a few similar cases reported here and in OpenWrt bug tracker for the MT 7981 used by Beryl AX, for instance WAN freqent disconnects on Beryl AX and What's wrong with the WAN port (MT7981) - For Developers - OpenWrt Forum.
I applied the workaround to swap LAN and WAN interface in Luci advanced interface (basically assigning WAN to eth1 instead of eth0, and swapping the ethernet cables) and I am not able to reproduce the issue anymore. Time will tell how reliable it is.
The root cause is unclear. Some of the bugs mention issues during speed negotiation with the upstream WAN router, because the default WAN port is expected to support up to 2.5G ethernet. The ones I've found are about the open-source drivers, and introduced in OpenWrt 23 so obviously don't apply to the Beryl firmware based on OpenWrt 21, but it seems clear that this Ethernet port is brittle.
I'd suggest adding an option in the "easy" interface to swap LAN and WAN, and documenting this as a known issue in the FAQ, so that the workaround is easier to apply without going through Luci.
I'm happy to run more tests or install alternative firmwares if it can help troubleshoot the root cause.