Tondo-
I have a couple of Roku extenders lying around here. One I formerly used before running my HDMI through the wall into the previously mentioned equipment cabinet. It just worked as intended. The other I ordered just as a test when people were having trouble placing orders, some time after getting the first one. I never had occasion to actually try using that one. Neither is a recent acquisition. The two are physically indistinguishable from each other, so I don't know which is which.
When testing with my guest room Roku Streaming Stick+ 3810 (much easier to get to than digging around in the equipment cabinet), the first one I tried did the same thing yours did. It felt as though it was making a solid connection to the HDMI jack on the Stick, but the TV could not detect a signal. Removing the extender and plugging directly into the TV restored the signal.
Curious now, I tried the other extender and it worked without a hitch.
I then tried the extenders on the HDMI cable going from my computer to my monitor. The one that worked with the Stick worked and the one that failed with the Stick failed.
As the two extenders look exactly the same I don't think the 4mm socket recess is the problem because it is the same on both. Evidently the connector at one end or the other, or the wiring connecting them, is faulty on the one that fails.
I don't recall seeing other complaints of failed extenders, so I've got to believe it's not all that common a problem. If it were me, I'd just write it off and concoct my own extender.