I'm testing the latest OpenWRT Snapshot (r20029-3c06a344e9), running the 5.15.50 kernel via testing mode on a WD N750, but observed the same SquashFS errors upon initial boot after flash.
Update: Possible improvement! After a reboot, no errors and there have been no errors for past 7 days. No issues on second reboot after 7 day uptime either. In the past on OpenWRT 21 and 22 I would see a few SquashFS errors immediately upon reboot.
I built the current firmware image from source using the July 7th snapshot, selected testing kernel and running 5.15.50. I'll keep monitoring and will report back.