End Dragon won't respawn on single player world

I've seen this issue before, but under different circumstances and from reading the comments, it looks like this needs to be a separate ticket?

Simply, when I go to the End and place an end crystal on each side of the return portal, nothing happens. I was able to respawn the end dragon once about a week ago, but now nothing happens.

I'm not running on Realms. This is a single-player world.
This is version 1.10.
I haven't touched any of the files in my minecraft directory/directories.

My world is quite old (created during the beta many years ago) but has been kept modern with each update. It's also quite large (almost a GB when compressed) so while I would like to attach it, I can't.