This seems to be the same bug as MCPE-63284

However, it didn't happen immediately.  We were successfully using the gateway with no issues for months. 
Today someone used the gateway and it doesn't work, it teleports you to 0,0,0.

Someone else used an elytra to fly out to the coordinates we expected to arrive at and everything looked fine, there were 2 end gateways (stacked on top of eachother (about 12 blocks higher that the main one), we didn't remember there were 2), but otherwise our build structures were still there around that end gateway.

The far gateway works correctly (returns to the main island), but the gateway on the main island still takes you to 0,0,0. 

Nothing we tried to do worked (including destroying our structures around the far end gateway, and building flat endstone around the area)

I've attached a clip of the falling into 0,0,0