If you add subscriptions back to back on the same contact record within a short period of time it was cause weird things like this. it takes about 10 mins for the automation to finish with one order. In some of these automations we have 3rd party integrations look at the last order/sub that was added to that account and make changes.
*Wait ~10 mins after you have paid an invoice to add another subscription/membership.
*If you added a subscription/membership and did not pay it, you can add another at any time without any issues.
Items that could be affected on the subscriptions and contact records - Dates, Order names, and Prices.