I have no Thunderbird extensions installed.
I sent this message (in blue below) with the subject shown. I received it too, as I was a recipient.
I then selected it (as you see above) and replied to it (replying to my own message, to add new info). I didn’t touch the subject line. I sent the new reply out. I receieved it too, as I was a recipient. Here is how I received it. Take note of the changed subject.
Update: Testing shows that the subject is being altered by Thunderbird as soon as I click on the original message and hit reply. The reply window appears with the subject altered.