Membership expires when created

Hi: Membership entity has been working fine on my site for months but a few weeks ago it suddenly exhibited the following bug.

Now, when a authenticated user "joins" a specific membership I have set up (ATHSS membership) the site creates a new membership for them as expected but shows the membership as being "expired". And it does not assign them the appropriate "premium" role.

What is odd about this is this happened today, and when i did it for myself it replicated the issue. And it has happened twice before that have been reported. BUT, in between there have been several users who have successfully created memberships that had the correct term (3 months) attached. In other words, sometimes it works and sometimes it does not.

For the purposes of debugging, only logged in authenticated users can see the "join" link, so this is unrelated to any issues about creating a Drupal account, registering etc.

I set this to "major" as it seems to be a bug at the very heart of the way I assume this is meant to be working. Could a recent Drupal core update have led to this issue by any chance?

Any suggestions?