After 1.7.10 the end portal animation changed. In its current state it warps and changes its facing direction to match the camera, when before it looked more like a window into another dimension.
It's hard to explain so I made a video:
MC-146364
End Portal Animation Bugged
Resolved
Invalid
0
(Unassigned)
Unconfirmed
Minecraft 1.13.2, Minecraft 19w11b, Minecraft 19w12b
Retrieved
I believe this bug started in version 1.8.