I have a Samsung Galaxy S10 Plus running the most up-to-date OS as of writing this message. When you receive a notification while performing a quick reply, the receiving notification will close the reply and the keyboard. Sometimes when you re-open the notification to reply part of the text in the reply you previously started is still there, while other times it's completely cleared and you have to start all over with your reply.
I first noticed this issue while watching YouTube videos and quick replying to my wife via text; if I received another text while in the middle of the quick reply, the incoming notification will completely close out my reply window. I then noticed it more often using my phone while working in IT... on a daily basis I will be assisting someone by watching a YouTube video or scrolling through information on a web page and then communicating with a user via quick reply (Text, Email, Teams, etc) instead of switching back-and-forth between apps; any time I received an incoming notification from the same app I'm replying to, the notification will cause the quick reply and keyboard to close completely.
I have been using quick reply since I first purchased the phone back in 2018. This issue was not always present and only recently started occurring within the past year.