That's great news!
I was thinking about the incompatibility problems with SC and OG GBA & GB Player. It could theoretically be the RAM chip that differs on different motherboard revisions. From what I have seen, most GBA SP motharboards have a NEC/BSI chips while OG GBAs and GB Players usually have hynix chips. This of course isn't a rule but we know that SPs don't have visual glitches and all (compatible) games seem to load.
So my hypothesis is that RAM chips in GBAs and GB Players might have a tad different inputs. Original games don't have problems with that, however when SC is loading the ROM, it has to be patched in some way due to its limits. And the patch only counts with the GBA SP/DS RAM. The ROM will load but it won't exchange information with the GBA correctly resulting into visual glitches, crashes or black/white screens.
It could also be the CPUs that are different - original GBA apparently has 128 pins while SP 156 pins. Inputs might be, again, different. Maybe the patches could be altered or maybe a "GBA mode" could be implemented so the cartridge will detect if it's in a GBA or an SP (or a DS) and that would change its behaviour in some way the AGBs and GB Players wouldn't have problem with anymore.
Again, this is just a hypothesis. I haven't done a research of any kind but I wanted to share this thought. Below I shared few photos of differences between different revisions of GBA SP, GBA and a GB Player. Just for reference.
EDIT: I just checked few more GB Player motherboards and they all seem to have CPU with 128 pins....
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