How to reproduce

1. Summon two entities with CustomName

   /summon zombie ~ ~ ~ {CustomNameVisible:1b,CustomName:"{\"text\":\"aaa\"}"}
   /summon zombie ~ ~ ~ {CustomNameVisible:1b,CustomName:"{\"text\":\"bbb\"}"}
   

2. Watch their name tags from both sides

In the screen shot, background of the "bbb"name tag should be behind the "aaa" name tag.