I had a similar problem. The body would reduce for all records except the last record. I resolved this by setting everything in the sub-summary to slide up. Not sure why this worked but it did. I did not have to do this in FM 11.
Might be because of the change from pixels to points. I had a few layouts like invoices with external tables that reduced perfectly in FM 10-11 but not at all in FM 12. After moving stuff around I realised that I had small objects that were the perfect size in pixels to allow resizing but after conversion those object seemed to be 1 pt to big.