Sorry I can’t recall where I read this advice (Tracy DiSabato maybe?) but I have tried cutting back Miscanthus in late spring/early summer. I had a garden in Connecticut at the time. The grass had just gotten too big and floppy, and the weather never cooperated that spring to find the extra time to excavate the darn thing and saw/chop/hack it into pieces. I was dubious, and the grass did not look great for the first week or two after its haircut — but the technique worked beautifully. The grass grew in well, stayed a manageable height and didn’t fall apart in late summer.