Just like mobs, end portals don't render when you get too far away from them. I don't think this has any noticeable impact on performance, as usually and reasonably, there will never be more than 24 blocks of end portal on the screen at once.
(This is also happening to chests, but I can see how it's needed there)
See attached screenshots.
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Actually, this is really intended. The end portal blocks actually contain block entities which have binded textures. Entities disappear when you go over 32 blocks away from them, so this is the reason why you don't see the portal from far away.