grdtrack (gmt5) - possible bug?

The manual says:
"You may append +iinc*[**u*] to set the sampling interval (append
appropriate unit); if not given then we default to half the minimum grid
interval, and *if geographic we select great circle distances in km as the
default unit and method*."

However, when I run:
gmt grdtrack -G${grd_file} -E${LON}/${LAT}+o${AZ}+l5
I get:
-118 33.5 -2.5 -0.0740850940347
.
.
.
-113 33.5 2.5 0.0141758602113

So the distances are in degrees, not in km, as I understand from the
manual. Why?
Also, it seems like that one can only specify +l in degrees, and not in km.
Why?

I also tried specifying the units in km with +i :
gmt grdtrack -G${grd_file} -E${LON}/${LAT}+o${AZ}+l5+i0.1k

However, I get intervals of 0.1 degrees instead of 0.1 km:
-118 33.5  -2.5 -0.0834747254848
-117.9 33.5 -2.4 -0.0538280569017
-117.8 33.5 -2.3 -0.0227038227022
-117.7 33.5  -2.2 0.0134101686999
-117.6 33.5 -2.1 0.0674700662494
-117.5 33.5 -2 0.12702396512

Is this a bug?

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