Comment 4 for bug 1846232

I believe this is due to
https://github.com/systemd/systemd/commit/4b151b71320bbee1549afcbad5554a40d90d63b4

and probably is
https://github.com/systemd/systemd/issues/12552

fixed by
https://github.com/systemd/systemd/pull/12574/commits
which prevents the automatic mtu bump when MTUBytes is specified

but i'm still uncertain about upstream's link_get_requested_mtu_by_stacked_netdevs() function, that bumps MTU automatically; I might be misunderstanding it, but I don't think it is needed or even correct - but I need to look more closely at it and haven't had time.