Also note when using retropie there is no issue. I think it's related to the power usage needed for recalbox at boot.

I use eneloop NiMH batteries. NiHM batteries deliver 1.2 Volts (all NiMH batteries do by the way <= correct me if wrong) but Alkaline batteries start at 1.5 volts but they gradually drop to less than 1.0 volts. NiMH batteries stay at 1.2 volts for most of their discharge cycle.

I think people using Alkaline batteries have more succes at stable booting recalbox.

Could it be recalbox draws more power "then necessary" at boot?

I've got the same issue with the batteries. Other people also have this issue.

There seems to be an issue with Recalbox 8 on the GPI Case and a Pi Zero 2 W.

When on batteries. On first boot after applying the image to the SD card it could be the setup runs correctly and recalbox is installed correctly. It could also happen to fail during the installation when on batteries. When powering via USB cable the system boots fine and setup completes normally.

Same issue occurs when turning the device on, on batteries after a successful installation.
When powering via USB cable the system boots fine, after turning on the device using the USB cable I can unplug from USB and keep it running perfectly normal on batteries. When booting on batteries, it won't boot.

Some people claim it's when using "low quality" batteries. I'm using eneloop batteries, tested using several other batteries (even new) but that is not the solution.

It appears there is a bug when using batteries only to power on the device?

A reddit post also exist concerning the problem.
https://www.reddit.com/r/recalbox/comments/rs06ck/weird_issues_with_recallbox_on_gpi_case_i_put_it/