I am not sure that it’s actually AxCrypt as such holding on to the USB drive. There are several mechanisms at work, one of them being Windows caching of data being written to a USB drive.
You can always try and really exit AxCrypt to see if it makes a difference. There are also various utilties and built-in windows functions to see just what program is locking a file or drive. Windows Resource Monitor and wholockme are two.