I’ve seen this mentioned a few times already: people wondering why certain communities don’t show up in their instance. You need to first search for that community via the magnifying glass icon and type in “[email protected]”
Notice the exclamation mark at the beginning. Might take a few seconds. This will teach your instance about the remote community.
Nice, I got it working with this type of mention [email protected]. But I want to see if I can convert the link form too, I will probably make a post once is done.