I finally found out where the problem is.
After downgrading OpenWrt to 24.10.0 an issue still persists.
But in OpenWrt 23.05.5 everything works flawlessly as it should.
I analised ppp protocol related packages in theese three latest versions. It appears that 24.10.1 and 24.10.0 both use the same version of ppp and ppp-mod-pppoe i.e. 2.5.1-r1, while 23.05.5 uses earlier version. It must be a bug in one of theese two packages.
Is https://github.com/openwrt/packages/issues the right place to fill a bug report on this?
I don't see theese packages are maintained there.