If you're inside an end gateway portal block with no teleport function in gamemode Survival, Creative, or Adventure mode, the texture will "disappear" periodically each (apparently) 1.950 seconds realtime or 39 game ticks for the length of (apparently) 0.050 seconds realtime or 1 MC game tick, causing a "flashing" which could be troublesome for people prone to epilepsy.

In 20w17a:

The "flashing" occurs only if the end gateway:

• is at least of Age 200 ticks (after the magenta, formerly yellow, beam changes to the purple beam)
• got a positive Age value > transmits a beam whilst an entity is inside it.
   

The "flashing" inside the end gateway occurs not:

• within the first 200 ticks
• with a negative Age value > when no beam is transmitted whilst an entity is inside it
   

The beam flashing can also be seen from the outside:
 

Additionally, there also seems to be a short "gap" (no beam transmission) below 1 game tick length, when the magenta (0-200 ticks Age) switches to the purple beam. During this "gap" though the end gateway texture itself is rendered.