Tested with the 1.15.2, the model brighten then before indeed, but still slightly dim on it like this comment.
I also checked the items from vanilla Minecraft and found that the original Minecraft items were actually affected too.
Workaround by rotating models in GUI still causes the model to darken in GUI but minimal.
First, I made an item that color is same as GUI:
[media]
Fill up with the "fake GUI item" and see the difference on the color:
[media]
Confirmed for 1.15.1
Get the same problem on me
rotation visual issue can reproduce in MC1.14.3
Video(, but command block version):
https://streamable.com/baxvu
I don't think it uses the int range, the entity_scores condition after this fix can even pass the score 16777216 in both conditions ✔ 16777216 and ❌ 16777217. (tested in 20w46a and 1.19.1-pre5)