Thunderbird spellcheck does not work sometimes when replying to message

I am using automatic spellcheck in **Thunderbird v68.8** on Debian Buster (misspelled words are automatically underlined) it works reliably when I create a new empty message but when I am replying to a message (either by clicking on "reply" or "reply all") the spellchecking doesn't always works. By "it doesn't work", I mean that not only is it not automatically enabled, but there is no way to activate spellcheck that i could find. **Is this reliability issue a bug? How can we have the spellcheck system always active, regardless whether it is new message or reply?**