The end gateway beam stutters during its animation because it only uses integer heights.
- Place an end gateway block
- Fly up around 45 blocks
- Toss an item down at the end gateway
- Move back so that you have an angle to see both the gateway and the beam.
- When the beam appears, it will not smoothly move between positions.
If no beam appears, make sure that the item went in and that the gateway block was within your field of view (MC-136756).
Alternatively, MC-136757 works well for reproducing this since you have a beam that varies in height at a rate you can control.
The code to render the beam in TileEntityEndGatewayRenderer uses TileEntityBeaconRenderer.renderBeamSegment, which takes an integer height. Changing that to a double would solve the problem entirely. Note that the rest of the code does correctly interpolate (including using partialTicks); it's only this last step that's incorrect.