As user Engineer Toast has said, by specifying justified text you have given Word
permission to arrange inter-word spacing as it likes, larger or smaller.
It seems like the word "occaecat" missed being included into the line by only
a few pixels, so that a minuscule reduction of some inter-word spacings in
the line was enough for getting it to fit in, which is an optimization that
Word knows how to do.
However, when you specify Left alignment, you don't give Word the permission
for varying inter-word spacing, specifying instead the fixed spacing that is
defined for the paragraph style. The word in this case no longer fits into
the line and is moved to the next line.
Working around this feature of Word is not simple.
One try would be to change the
Font character spacing
of the paragraph, although this might be too strong a change.
Otherwise, you could manually choose one or more blank space
character(s) in this line and reduce their font size to make them smaller.
You only need to make a very tiny difference to avoid the problem for this
paragraph, and this shouldn't affect the display in Justified mode.