Just to be utterly unambiguous, this is someone who shows up in your blocklist but doesn't show an option of unblocking or viewing the conversation. I've tried to find a way of reproducing this, but so far no luck.

Since the blocklist is purely within Textra rather than a system feature I expect that if you set another SMS app as default (e.g. the one that came with the phone) you'd be able to see these missed texts. But of course that's not what you want, or else you'd be using that app already.

Have you tried sending this person a message yourself? That will start a conversation with them, and maybe that will change the status of this "blocked but not unblockable" conversation. Of course it might be that it won't let you message a blocked recipient (I've never tried), but in that case perhaps it will ask whether you want to unblock. This is just an idea, no idea whether it will work.

The one certain way to unblock it is to clear Textra's data completely (Settings > Apps > Textra and select "clear storage"). That will erase Textra's blocklist, as well as removing all of your other customisations, so will require setting up from scratch again afterwards (including re-blocking those conversations you want blocked). But it won't erase your SMS because those are not owned by Textra (an SMS app is just an interface to the message service, there's a separate system app responsible for storing messages independent of which SMS app you use). If you are worried about this set a different SMS app as default before clearing Textra's data: that way Textra won't have read/write access to your messages when you wipe it.