That is strange, dropout should be a straight copy.  Also, it’s considered a neutral op for mixed precision (inputs aren’t changed).  Are you sure dropout is most recent operation that occurred?  I’d put a print statement right before and after the offending dropout function call to be sure.
In any case, if you’re using Apex Amp, you should be aware that will soon be deprecated.  I’m working on native Pytorch support for mixed precision, targeting the upcoming 1.5 release:
https://github.com/pytorch/pytorch/pull/32140 3
https://github.com/pytorch/pytorch/pull/33366 2
PRs have upstream approval and are undergoing end-to-end testing locally.  It may be preferable to wait for the native integration, and see if that resolves your issue.