Custom role not assigned when product purchased

We can't get roles to assign when a product is purchased. It used to work a few UC versions ago. We stopped using UC for roles for a while, upgraded to 2.4 and now want to again have a custom role assigned when a particular product is purchased. But now nothing happens regarding role assignment -- we must do it manually.

Everything we can check indicates it should work. Only ONE role is involved. It exists in Drupal and does what it should if we manually give it to a user.

This role is assigned to the product (shown in Features section). But when we get an order for this product, everything works fine EXCEPT role assignment -- it simply does not happen. We use PayPal Pro, and payment flows, e-mail to customer goes out, etc.

We checked cron and it's running, according to Apache and Drupal logs.

To diagnose, we created a new product, from scratch to avoid any old baggage, gave it the role as a Feature, and did some test purchases of it. But the role doesn't get assigned to the user.

We waited many hours for UC to assign the role, get no errors but no action either.

As an experiment we created a new CA that looks for Order status = Complete OR Order status = Payment received. We used OR, not AND, to have two points in the order flow that would get the role assigned. But nothing happens.

We also left UC's default role "renewal" and all other default CA's in place, hoping at least one of them would do something about the role assignment, but it doesn't happen.

We created new orders to try and trigger the actions, but the test user's roles never change.

We don't know what else to check...

Side question, not the case with our test, but we wonder: UC seems to connect with user's Drupal record via user's e-mail address rather than Username -- is this accurate? If so, it is possible (fairly common) for user to change the e-mail address. When this happens, will it break UC in some way?