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.