I just changed a preformatted to normal paragraph style when the composer seemed to remove one of the leading spaces of that particular line. Sending the message shows it there, at least when I send early, without editing anything else.

Steps:
a) new composer, plain text mode, paragraph style Normal
b) type:111<enter>
c) change paragraph style to Preformatted
d) copy & paste the following text between double quotes, but without them; only
   note that's one long line without any \n or \r and contains 3 leading spaces:
   "   aaa bbb ccccc ddddddddd eee fffffffff gggg hh ii jjj kk ll mmmmm nnnnnnnn oooooo"
e) change the paragraph style to Normal

Actual result:
|111
|  aaa bbb.... (only two leading spaces)


Expected result:
|111
|   aaa bbb... (three leading spaces, as it was the state before changing the paragraph style)

Note: The paste is not important, I can reproduce it when typing the text as well.