This bug affects 21w03a, not the latest release, 1.16.5
Putting the glow effect doesn't work in 21w03a on any entities. Enter the command: /effect @s glowing 100 1
Note that you have the effect particles, and you have the message in your inventory with the Glowing effect. You can also use a bow with spectral arrows or dispense spectral arrows from a dispenser, but you or any entities will not glow.
Linked issues
Comments

Duplicate of MC-204782
Yes, I am on a MacOS but the bug only seems to appear on snapshots, not releases.

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Are you on a MacOS?