bug tracker woes/misunderstanding

[Drew Adams]

Someone replied to a parenthetical comment in a bug report (#8667).
I replied.  Since this side discussion was off-topic for the bug, I changed "RE"
to "OT" in the Subject line.  But I left the bug number and the rest of the
Subject as is.

The bug tracker created a new bug:

bug#8670: OT: bug#8667: 24.0.50;`bounds-of-thing-at-point' returns (N . N) for
`comment'

I thought it would keep the same bug # since the # was in the Subject.

Is this a tracker bug or the intended behavior?  What can we or can we not
change in the Subject line, without having a new bug created?