The Remote Getaway advancement is not granted in some situations. It's hard to say for sure what's going on, but I think that the advancement check is being skipped when the player uses an ender pearl to teleport to a position that would put them partially inside the gateway. This can be reproduced with some difficulty by throwing an ender pearl at the intersection of the gateway block and one of the surrounding bedrock blocks, as shown in the video below. I was unable to reproduce it by teleporting myself inside the gateway using /teleport.

This issue is related to MC-262469- in fact, the core issue may be the same.