If the extension finds a user and then you try to open that profile using your home instance, but your home instance doesn't know that user yet.
Most fedi servers like Mastodon (it seems) will not load that user just from visiting the url, you need to actually fetch the user (by for example performing a search with that username).
Otherwise it will just load a 404
for example (random, working username on a remote instance. My instance has never seen this user before)
https://en.osm.town/@C_J@tech.lgbt
The substitoot extension might be able to fetch these users in the near future, but It's probably a feature that should also be considered for this extension on it's own
https://github.com/virtulis/substitoot/issues/4
If the extension finds a user and then you try to open that profile using your home instance, but your home instance doesn't know that user yet. Most fedi servers like Mastodon (it seems) will not load that user just from visiting the url, you need to actually fetch the user (by for example performing a search with that username). Otherwise it will just load a 404 for example (random, working username on a remote instance. My instance has never seen this user before) https://en.osm.town/@C_J@tech.lgbt
The substitoot extension might be able to fetch these users in the near future, but It's probably a feature that should also be considered for this extension on it's own https://github.com/virtulis/substitoot/issues/4