I am currently using the S10 5G and updated to latest software version, there is a problem with my text messages when I get notified I usually slide it down and type whatever I reply and click the send button. Leave it be then when I check my phone it's still showing me the notification I had from a previous text and when I click into my texts there are no texts being sent and this happens quite often and I rely on this as it's a quick reply texts.
The normal function of the text messages when you reply in the notification panel is to show you the message you received and the reply you sent through the notification panel. You should still be able to see the message and the reply in the Messages app when you open it. Are you saying they are not appearing in the Messages app?
When I have replied in notification panel on text I hit send. When I check my messages app then it the message that I replied to is not there and not been sent or not appearing at all.
Are you using the stock messaging app? Have you tried clearing the app cache? https://www.samsung.com/us/support/answer/ANS00077611/