Me too, I reported my findings regarding this at the CE 21.1.1-ng thread.
Here’s what I found out for latest stable build: CoreELEC 21.1-Omega Amlogic-ng Discussion - #148 by unmoved6125
In addition to this, the latest nightly build has perfect playback (including FF/RW) when opening FEL movies’ BDMV/ISO via the index file or the mpls file. You might want to try that and see as well.