Bruh, FTTH is Fibre to the home, what in the world is “plain” FTTH? Unless you live right next to AT&T’s NOCs, it is a PON, no way your ISP can afford AON to millions of customers.
It can be GPON or EPON or XG-PON or XGS-PON or 10G-EPON.
Forcing 1280 IPv6 MTU is a poor solution. I don’t know why you’re resistant to fix the actual MTU on the underlying ethernet’s interface like I described and ensuring sufficient padding for any future RFC4638 from your ISP.
Underlying Ethernet
Leave RouterOS to auto-negotiate with the access concentrator based on the new ethernet MTU + jumbo frame negotiation on the PON link
Having proper MRU of 1500 even if ISP caps MTU due to no RFC4638 ensures incoming traffic is unfragmented and therefore full potential performance excluding choking etc on ISP/Local link.