Slight indentation with non-regular (bold/italic) font in groff

I am new to groff and noticed a slight indentation if the first word of a paragraph (or new line) is set in a non-regular font. In the following example, this happens only in the second and third paragraph, but not with words appearing at the beginning of a line after groff starts a new line itself (in my example, walk in the first paragraph, and girls in the last):

-groff
.fam H
.PP
I see a red door and I want it painted black
No colors anymore, I want them to turn black
I see the girls
\fB
walk
\fR
by dressed in their summer clothes
I have to turn my head until my darkness goes

.PP
\fB
I
\fR
see a red door and I want it painted black
No colors anymore, I want them to turn black
I see the girls walk by dressed in their summer clothes
I have to turn my head until my darkness goes

.PP
\fB
I see a red door and I want it painted black
No colors anymore, I want them to turn black
I see the girls walk by dressed in their summer clothes
I have to turn my head until my darkness goes
\fR

.PP
I
\fB
see
\fR
I see a red door and I want it painted black
No colors anymore, I want them to turn black
I see the
\fB
girls
\fR
walk by dressed in their summer clothes
I have to turn my head until my darkness goes

The output looks like this: Why are the second and third paragraphs indented, and how can I avoid this indentation?