We've seen the same between Juniper and Arista boxes in the same rack
running at 100G, despite cleaning fibres, swapping optics, moving ports,
moving line cards, e.t.c. TAC said it's a non-issue, and to be expected,
and shared the same KB's.
Just for extra clarity off those KB, probably has nothing to do with vendor interop as implied in at least one of those.
You will see some volume of FEC corrected on 400G FR4 with the same router hardware and transceiver vendor on both ends, with a 3m patch. Short of duct taping the transceivers together, not going to get much more optimal than that.
As far as I can suss out from my reading and what Smart People have told me, certain combinations of modulation and lamda are just more susceptible to transmission noise, so for those FEC is required by the standard. PAM4 modulation does seem to be a common thread, but there are some PAM2/NRZs that FEC is also required for. ( 100GBASE-CWDM4 for example. )