not sure what you mean by imported the key. I don't know that I have ever imported a key in any of my changes to new versions of MC. I think I followed the same steps for my bsmt pi, which I used the instructions in MC 26 linux on how to upgrade from MC25. For my family room pi I followed the instructions in MC 27 linux.
Here is the output from a few days ago apt-get update, from the problem pi.
pi@raspberrypi:~ $ sudo apt-get update
Hit:1
http://dist.jriver.com/latest/mediacenter buster InRelease
Hit:2
http://raspbian.raspberrypi.org/raspbian buster InRelease
Hit:3
http://archive.raspberrypi.org/debian buster InRelease
Reading package lists... Done
pi@raspberrypi:~ $ sudo apt-get dist-upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
When, after making no changes other than the audio options set to not play fromm memory (which I think is totally unrelated), here is what I get today:
pi@raspberrypi:~ $ sudo apt-get update
Get:1
http://dist.jriver.com/latest/mediacenter buster InRelease [3,612 B]
Get:2
http://raspbian.raspberrypi.org/raspbian buster InRelease [15.0 kB]
Hit:3
http://archive.raspberrypi.org/debian buster InRelease
Get:4
http://dist.jriver.com/latest/mediacenter buster/main i386 Packages [704 B]
Get:5
http://dist.jriver.com/latest/mediacenter buster/main armhf Packages [688 B]
Get:6
http://dist.jriver.com/latest/mediacenter buster/main arm64 Packages [642 B]
Get:7
http://dist.jriver.com/latest/mediacenter buster/main amd64 Packages [678 B]
Fetched 21.3 kB in 3s (6,377 B/s)
Reading package lists... Done
Weird to me, but whatever, I am now getting the updated versions. Now at version 27.0.21 I expect but it won't open.