I had basically the same experience on December 2. I had originally booked GRU-FRA-WRO, but the afternoon FRA-WRO was cancelled and was accommodated on FRA-WAW-WRO. To my surprise, as I was getting off the plane from GRU I noticed that the flight to WRO was operating. Luckily my request to be put back on the direct flight to WRO was met without any question and within 2 minutes I had a new boarding pass.

I sympathise with OP, especially given the current situation 7 hours at FRA could not have been fun.