The following warning in the log

Package silence Warning: There were 6 warning(s) and 0 error(s).
(silence)                I've killed 5 warning(s) and 0 error(s).

is displayed in quickfix window by vimtex as

Package silence Warning: There were 6 warning(s) and 0 error(s). and 0 error(s).

it seems that in the attempt to join the multiline message, not only the leading (silence) and spaces were deleted, but everything until (and including) the warning(s).