Sorry, just saw MC-107824 (I'm bad in finding bugposts >;[) - will add info in there, if needed.
This can be resolved as dupe.

From what I know currently, purple end gateway portal block beams should only occur if the gateways are created or set freshly, or if something teleported (when the corresponding gateway block's beam shines yellow, until 16w39a), or if you reset their Age via command-means.

However, I just found by chance they emit a purple beam each 2 minutes (counting from when it was set/its Age; tested in 16w44a); I have never seen that behaviour in previous versions and am currently not sure if I just overlooked it, but if this is a newer bug, I might go back to see in which version it started to occur.

I quickly also tested it in 1.10.2 Vanilla, and there the beam does not occur within a timespan of about 6 minutes, so I'm assuming it's really something that changed throughout the 1.11 snapshots.

Video that proves the behaviour in 1.11 (16w44a) https://youtu.be/bea6rbEkGDE

I fiddled a lot with end gateway portal blocks in the 1.9 snapshots, release and 1.10.x release versions, so I would really wonder if I overlooked that behaviour back then, but it's always a possibility, although my quick test in 1.10.2 does say otherwise.

Steps to reproduce
1. setblock single gateway blocks or fill an area with them

/fill ~-4 ~-2 ~-4 ~3 ~2 ~3 end_gateway 0

2. Wait for 2 minutes, and you should see a purple beam occuring again.

If you set the end gateway portalblocks manually/single, the beam will occur according to their age, e.g. if you set 2 blocks with 5 seconds apart from each other, the beam of the second gateway will occur 5 seconds after the first one.

I also tested it by filling an area with end gateways in 1.10.2 Vanilla, went back into that world in 16w44a, and the gateway portalblocks emit that purple beam after 2 minutes. So I'm assuming this would happen also in other worlds, as soon as they update to 1.11, unless it gets fixed before.