That feature works fine in release version, so maybe there are other possibilities, such as addons. You did not mention OS (e.g., Windows, Linux), or Thunderbird version number.

I have no addons, Linux mint and it is the latest version 102.11.0. The problem started after the latest thunderbird update. This have never happened to me before when I reply to a mail. I have tried for a day and a half to find an answer and now it seems to work as it has done before. But it does not help as I needed to reply yesterday.

....now it seems to work as it has done before. But it does not help as I needed to reply yesterday.

Is it possible you restarted Thunderbird in between and it's fixed itself?

It must have been a real nuisance, but I'm assuming you could copy paste the received emails into the Write window or use 'Edit as New Message' to get around the issue or even use 'Forward' - not quite perfect but it would be a temp workaround.

However, it's good to know the problem is resolved.