To use the proprietary MediaTek WiFi driver they’ve downgraded from OpenWrt 23.05.0-rc1 all the way to OpenWrt 21.02.0-rc1. But OpenWrt 21.02.x reached it’s end of life in May 2023 
4.5.6 stable release:
OpenWrt 23.05-SNAPSHOT, r23001+721-38c150612c
4.5.7 snapshot:
OpenWrt 21.02-SNAPSHOT, r15812+1067-46b6ee7ffc
Technically the builds are newer than 23.05.0-rc1 and 21.02.0-rc1, but the commit ID (e.g. 38c150612c) shows us where GL.iNet started their copy of OpenWrt. And of course they’ve then applied their own fixes and cherry picked some changes from newer OpenWrt snapshots too.
Stability with this approach is questionable though, since they forked OpenWrt 23.05 before it was stable and they haven’t kept their OpenWrt fork in sync with the OpenWrt 23.05 repository, so it’s missing dozens of fixes. For example, with firmware 4.5.6 if you open up /lib/netifd/wireless/mac80211.sh and go to line 148 you’ll notice that it’s missing this change from January 5th. But if you go to line 330 you’ll find this change from January 9th.
Have you tried using official OpenWrt snapshots, just to see how that compares? Because if it works with official OpenWrt snapshots then GL.iNet could fix the problem by rebasing their fork on a newer OpenWrt snapshot.
Rebasing the GL.iNet firmware on a newer OpenWrt snapshot might improve the overall stability, but it won’t do anything to fix the 2.4GHz issue. Although, now that we’ve got a firmware with the proprietary MediaTek WiFi driver to compare against someone could file an issue report to the mt76 repository.