Drupal 7.69, CiviCRM 5.26.1
I had a similar issue (but seem to have identified something different) a few months ago.
Trying to send a mailing to a group, 'Donors'. This group is not a smart group, but it has child groups which are smart groups. I've sent emails to this group many times before. When I select the group, it says no recipients:
I then click save draft and re-enter the mailing, and it's stuck on 'Estimating'.
This is what I see when I click on the wrench:
This seems to happen for some groups but not others. I've emailed all these groups in the past.
Flushing the cache did not work. Any ideas on how I can fix this? I really don't want to have to rebuild all the groups all over again, this one in particular has a lot of different sub-groups within it.