Prior to 1.10 I was able to respawn the End Dragon. I recently mined one of the obsidian pillars to gain resources naturally thinking this was the cause of not being able to respawn the dragon I replaced the obsidian but to no avail still no dragon. I tested this on a single player world and was able to respawn the dragon no problem. Not sure why but realms world doesn't want to respawn it. I tried restarting the game numerous times and still no use.

More info: https://bugs.mojang.com/browse/MC-103497?focusedCommentId=312883&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-312883