I no longer have the world, but i am unable to recreate this bug in 1.16.4
I maneged to reproduce it without commandblocks
https://www.youtube.com/watch?v=ASYu5b3bCy8&feature=youtu.be
(vid is too big to upload here)
This also happens with just one enderdragon
I no longer have the world, but i am unable to recreate this bug in 1.16.4