The validity of a Flexi German Rail Pass is always one month so there is nothing wrong.

https://www.interrail.eu/en/interrail-passes/one-country-pass/german-rail-pass

Thanks for your quick reply. This is very strange: I see that there is indeed no 2 month option via the link you posted. However, I do have the receipt that says 2 months 10 days. I bought it specifically for this reason…

Ok then best thing is to get in touch with customer service. (due to a big amount of request, they need some time to help you) 

Submit a request – Eurail Knowledge Base (zendesk.com)

Maybe this helps, as I see an exchange to global as only ALTern: IN DE=germany till  31/8 you can buy for just 9€ a month-till last day of month, a ticket that is valid in ALL of DE in ALL transit-incl bustrammetro, but NOT the sleek white IC/ICE for long distance. This also means overcrowded trains and many more changes for longer trips-but as such its possible. Then you start usijg the GRP later on-and let it end in sept.