The Bug:

Before 22w15a, Allays glow had a soft transition that made it fade in and out. After this snapshot, the Allay now permanently glows at one solid, unchanging light level. There was no mention of this in the changelogs  so I assume it's unintended.

How to Reproduce:

1. Summon an Allay with NoAI:1 pre-22w15a. You can see the animation of its glow fading in and out.
2. Do the same in 22w15a. It will no longer fade.

The videos below demonstrate this.
Curiously, this fade that previously existed seems to match what the unused allay_dust particle does when in low light levels.