If I recall correctly, that’s because HEE has a way of regenerating the entirety of the end after a certain ritual: this was both added as a way to kill the dragon again before end crystals were added (I think that was around either 1.8 or 1.9) and to add replayability to the End dimension.
The hardcore ender expansion wiki has more detailed information about the process here
And there’s also an entry page for in the ftb wiki here
https://ftbwiki.org/Temple_Caller
Hope that helps!