After converting an old save to 1.17 RC-1, all existing end portals in the world had the same, glitched and stretched texture when compared to the expected texture. 

Steps to Reproduce:

I was unable to find consistent methods to reliably produce this bug, however it appears to be consistent with specific worlds. The attached world below appears to consistently have this issue. Graphics hardware does not appear to change the behavior; my friend and I tested this and we both got the same results. The attached world was originally generated in 1.14.4, but it also consistently worked in a world originally generated in 1.16.1.

The steps to reproduce this with the specific world attached below are as follows:

1. Download the world file attached and add it to your saves.
2. Load Release Candidate 1 and open the world.
3. The portal's textures will appear stretched and bugged out.
4. Enter the portal and kill the dragon. (Using commands is fine.)
5. The exit portal will also look bugged out.