The bug

Normally, when the outlines of several glowing entities overlap, their outlines are combined into a single outline. However, when the glowing entities are translucent, this often does not occur; the outline of the translucent entity will render on top of the outlines of some glowing entities and behind the outlines of some other glowing entities. This can cause the shape of the outlines to be inaccurate and strange. The order in which the outlines render often changes when you reload the world.

To reproduce

Detailed reproduction steps for two manifestations of this bug are listed below.
Inaccurate Outline Shape

1. /fill ~ ~ ~ ~ ~5 ~80 minecraft:black_concrete

2. Stand on the east (positive x) side of the wall. Stand close enough to the wall that the below commands will summon entities on the other side of the wall.

3. /summon minecraft:slime ~-5 ~ ~ {NoAI:1b,Glowing:1b,Size:3}

4. /summon minecraft:sheep ~-8 ~ ~ {NoAI:1b,Glowing:1b}

5. Notice that the outline of the sheep hasn't merged with the outline of the slime.
   

6. /summon minecraft:slime ~-11 ~ ~ {NoAI:1b,Glowing:1b,Size:3}

7. Notice that the shape of the sheep's outline now looks inaccurate and strange.
   
8. Optional: Run the following commands to see that some edges of the inaccurate outline shape become blue and the other edges become red.

   /team add RED

   /team add BLUE

   /team modify RED color dark_red

   /team modify BLUE color blue

   /team join RED @e[type=minecraft:slime,distance=..15,limit=1,sort=nearest]

   /team join BLUE @e[type=minecraft:slime,distance=..15,limit=1,sort=furthest]

   

9. Also optional: Reload the world several times and notice that the order in which the outlines are rendered (and thus the shape of the sheep's outline) often changes.
   
10. Also optional: Repeat all of the steps above, except use magma cubes instead of slimes. Notice that the outlines always merge as expected. The strange behavior that happens for slimes is caused by the slimes' translucency.

See Golem Cracks or Horse Fur Patterns Through Other Glowing Entities
Golem cracks and horse fur patterns are translucent textures, so they are affected by this bug. (The fact that golem cracks and horse fur patterns are outlined at all is MC-173834 and MC-220109, respectively.)

1. /fill ~ ~ ~ ~ ~5 ~80 minecraft:black_concrete

2. Stand on the east (positive x) side of the wall. Stand close enough to the wall that the below commands will summon entities on the other side of the wall.

3. /summon minecraft:polar_bear ~-5 ~ ~ {NoAI:1b,Glowing:1b}

4. /summon minecraft:iron_golem ~-8 ~ ~1 {Health:74f,Glowing:1b,NoAI:1b}

5. /summon horse ~-8 ~ ~-1 {Variant:773,NoAI:1b,Glowing:1b}

6. Notice that the golem's cracks and the horse's fur patterns do not merge with the outline of the polar bear (), but the regular external outlines of the golem and the horse do merge with the outline of the polar bear ().
   
7. Optional: Reload the world several times. Notice that whether or not the cracks/fur patterns render in front of or behind the polar bear often changes.