When a villager is glowing it has black dots near it
To reproduce
/summon minecraft:villager ~ ~ ~ {Glowing:1b,NoAI:1b}
/summon minecraft:wandering_trader ~ ~ ~ {Glowing:1b,NoAI:1b}
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I was able to reproduce with Windows 10 with a AMD Ryzen 7 2700X Eight Core Processor CPU and Radeon RX 5500 XT GPU

Able to reproduce in 21w17a with an AMD Ryzen 3 3100 and Nvidia RTX 2060 (Driver 466.27). However, the dots flicker whenever I move, as opposed to appearing while stationary.
EDIT: appears to also affect Wandering Traders.
I am actually able to reproduce this issue, it's just that this happens quite rarely and randomly. Also affects 1.16.5 along with 21w18a.

Does this only affect villagers and wandering traders for you, or are all the mobs listed in MC-169921 affected?
Can confirm in 1.17.

Affects 1.17.1 RC-1
Can confirm in 1.18.1. You can use the following commands to make reproducing this much easier.
/summon minecraft:villager ~ ~ ~ {Glowing:1b,NoAI:1b}
/summon minecraft:wandering_trader ~ ~ ~ {Glowing:1b,NoAI:1b}
Can confirm in 1.18.2.
Can confirm in 1.19.
Can confirm in 1.19.1.
Can confirm in 1.19.2. This issue isn't environmentally exclusive.

i can confirm in 23w51b
Cannot reproduce, maybe macOS specific