i  am able to replicate this. i am on the latest version 1.16.2.  so if i am close to a pane or block of stained glass (any colour) about 1-3 blocks away the end crystals  show but as soon as i am 4 or more blocks away the end crystals disappear and reappear as i move closer i have restarted my game(and launcher) and it still happens. on regular glass and panes it remains visible. before i started this i did check the known issues and was unable to find this one. also my game was unmodified or pure vanilla minecraft