Hey  @RoadTurtle,

It is possible that the devices are still connecting to the Router and are seeing the RE as interference. I would recommend hard rebooting the devices you are seeing the lessened connection on.

A common problem with REs is their placement so keep it in mind even though you have tested in another room. REs should be placed halfway between the router and the area you would like your network extended to. 

It could also be the result of which band is being used. With the RE so much closer, your devices may be connecting via 5GHz instead of 2.4Ghz, which was most likely being used due to the coverage of 2.4GHz networks indoors.

You may consider setting the RE up to use a different SSID to see if the behavior improves.