Font change in plot -l string not correctly intpreted in legend command

Using gmt 6.4 I came across the problem that the font change in the first legend string was not correct

# generate legend
rm -f legend.txt
cat <<- END > dummy.dat
-128 -9000
0 -9000

gmt begin a
   gmt basemap -JX7/5 "-R0/1/0/1" -Bewsn
   gmt plot dummy.dat -Wthick      -l'@%1%m@%% (rot. feedb.)'
   gmt plot dummy.dat -Wthick,blue -l'@%1%m@%% (rot. pot. only)'
   gmt plot dummy.dat -Wthin    -l'm@-x@-'
   gmt plot dummy.dat -Wthin,-  -l'm@-y@-'
   gmt plot dummy.dat -Wthin,.  -l'm@-z@-'
   gmt legend -DjTR+o0.2c -F+gwhite --FONT_ANNOT_PRIMARY=8p,Helvetica -Tlegend.txt
   # modificiation due to bug in legend function.
gmt end show

resulted in a not correct setting of the legend. I echoed the legend entries to legend,txt and they read,

S - - - - thick,black - @%m@% (rot. feedb.)
S - - - - thick,blue - @%1%m@%% (rot. pot. only)
S - - - - thin,black - m@-x@-
S - - - - thin,black,6_3:0p - m@-y@-
S - - - - thin,black,0.75_3:0p - m@-z@-

So only in the first line @%1%m@%% was changed to @%m@%. (I want to set the m in bold)
The dummy I did to use the legend information later in a different plot.