Check their true expiration date within CBSubs > Subscriptions by clicking the # under the ID column and ensure they're withing your 3 month renewal window. It's possible they maybe outside of that window. All calculates in CBSubs will be with UTC (same for storage). Their account being disabled is normal if you don't allow free registrations without a subscription.

Does the plan require a usergroup or view access level they no longer qualify for? If so they'd lose access to the plan and be unable to renew.

I'm not sure what more to suggest. Recommend creating a test user, subscribing them to your plan, then within CBSubs > Subscriptions modify the subscription date, expiration date, and set status to expired. Next login as that test user to see if it allows you to renew. This should allow you to reliably retest the scenario. If you can confirm then begin reviewing the configuration of your plan carefully as something is blocking the renewal (likely some sort of access permission).