.config/aerc/aerc.conf(tried on aecr 0.7.1) have the following:
open aerc and type
:filter <whatever> – it does not matter what you try
select any message and try to open it
it does not open the message, no error message or anything
open the message and continue working as normal
In my experience, what it actually happens is that the list of messages is not updated apparently: you keep seeing the same list as before filtering. However, if you move your cursor down you'll see that the cursor stops at some point before the end of the visible message list: I've checked and it seems to be precisely the line corresponding to the number of messages returned by the filter, which indicates that, somehow, the filter has actually worked (the inner list of messages has been filtered), but however the shown list of messages hasn't been updated as a result. Am I being able to express it properly? Just try with a filter for which you know there's a certain number of messages.
the threading feature is quite experimental and it is possible that filter/search does not work properly with it.
In fact, since the threading-enabled has been discarded in favour of the toggle-threads, I haven't experienced this bug anymore.