The Bug
When tossing items/pushing mobs into an End Gateway, the expected Magenta beam occasionally does not appear. This has been noticed much more frequently when these entities enter via the top or sides of the end gateway block (beam appears consistently when tossing items up from the bottom).

The items/mobs are transported properly and can be found at the gateway destination

The consistency from dropping entities via the sides/tops and a beam appearing are much less compared to that of the bottom causing a beam most of the time.

How to Replicate

1. Start a Default Minecraft world, preferably with cheats, and travel to the End via the stronghold in creative mode.
2. Kill the dragon via the /kill @e[type=Ender_Dragon] command
3. go to the spawned end-gateway and break the blocks covering the gateway portal blocks top and bottom.
4. Toss items/entities into the top of the portal, as well as the bottom, and notice the beams appearance or lack thereof.

Note: Shulkers cause the appearance of a beam 100% of the times checked.

Attached video shows footage of testing. Note the wool all appearing at the gateway destination, ruling out any falling into the void