Happens all the time. FMA fans notoriously review bomb any anime that passes their decade old show.

On MAL and temporarily, yes. From memory, I can think of two anime that were temporarily ahead of FMA:B.

Kaguya-sama's third season was ahead of FMA:B when it was airing in 2022. It now sits on 12th with a score of 9.02.

Pingu in the City was memed to #1 shortly before its first episode aired in 2017. It now chills in the mid-tier with a score of 6.52 on 6596th.

However, I think Kaguya was only barely ahead of FMA:B at its peak. Meanwhile Frieren is at 9.18 currently while FMA:B in 2nd place is on 9.09. That is a full 0.09 lead, which is over 10% of the way from FMA:B to a theoretical clean 10.0.
Another way to put the lead into context is by doing the next 0.09 step down from FMA:B, which lands us cleanly at 9.00. An anime with a score of 9.00 would be at 13th, between Kaguya-sama 3 (9.02) and Fruits Basket: The Final (8.98).

Edit: With the final episode done, it's now sitting at 9.34. I will assume it's gonna drop a bit again over time, this is still an absolutely insane score. 27% of the way between FMAB and perfect 10.0. Going from 9.34 is now a 0.25 step down to FMAB, the next 0.25 step would be down to 8.84, which would be a tie with Kimi no Na wa at 28th.

I can only repeat again how mindblowing this is. The difference between Frieren and FMAB is the same as the one between FMAB and Kimi no Na wa. And that is ignoring how much harder every 0.01 gets the closer you get to 10.0.

Briefly, I believe.