Happens to me too. Have to hide the keyboard to hit Post as well
nostr:npub1gcxzte5zlkncx26j68ez60fzkvtkm9e0vrwdcvsjakxf9mu9qewqlfnj5z weird bug on Samsung replying. Post button gets stuck in the notification area and can't click it. Closing the keyboard fixes it but had to figure that out.
Discussion
Also, this only happens if the note you are replying to is really long. Otherwise, the preview doesn't get in the way. Probably some margins just need to be adjusted