I think cutting it now would ruin the shape of the grass. It may still bloom but it wouldn't look good. I'm wondering why it is flopping. Did it get too much fertilizer or is the soil too rich?

Well I went ahead and cut my Miscanthus to three feet tall -- and it looks horrible! I am seeing some new growth so I hope in a few weeks it will look decent. I have several others that I did not cut back so I will compare them all come September. As for my soil, I do not fertilize but I mulch each spring with a dark cedar mulch that our local provider calls "special blend." This mulch is mixed with cow manure and is the likely cause for the mid-summer floppiness.

In wet summers, Miscanthus (and most of the larger grasses) tend to be floppy when compared to dry summers. It often also is an indication that the plant could benefit from dividing.

Plus there are some cultivars that do not have a strong constitution and are a bit floppy by nature, M. sin 'Sioux' for example.