The bug

If an entity has the glowing effect, changing its data with the /data command will make the outline disappear; the effect is not removed altogether and the outline will reappear the tick after the command has been used.

This bug is more noticeable if the entity has its data changed every tick: doing so will render the outline constantly not visible.

How to reproduce

1. Spawn a chicken
2. Give the glowing effect to the chicken:

   /effect give @e[type=chicken] glowing 999999 0 true
   

   → The glowing outline is visible on the chicken

3. Using a data pack function, repeat this command every tick to change the chicken's data:

   /execute as @e[type=chicken] run data modify entity @s EggLayTime set value 2
   

   → The chicken is no longer glowing

Using /data merge instead of /data modify will give the same result.