Has anyone else seen Messenger change a conversations font size from the
default 10pt, to 12pt or set bold in mid conversation without being set
by the user?

For the font size, suddenly every other line or so will be the default
10pt, but all the others will be 12pt.

Or if its not the font size, it will just go bold.

Both can be 'fixed' by closing the conversation and reopening it.
I can't remember if it was in 1.0.x, but I've seen it since 2.0 and we
are currently running 2.0.2.