I wanted to add an option in the kernel grub command line with YaST. For the
test, I add the option "dummy"

I already had problems changing the default options on this way, but didn't
found why.

Previously I edited the /etc/default/grub file.

I did what I do most of the time: add # in front of the old option line then
add a new line below. Here what was the file before using yast:
(...)
GRUB_TIMEOUT=8
#GRUB_CMDLINE_LINUX_DEFAULT=nomodeset
GRUB_CMDLINE_LINUX_DEFAULT=nomodeset
# kernel command line options for failsafe mode
GRUB_CMDLINE_LINUX_RECOVERY=single
GR
(...)
and after:

#GRUB_CMDLINE_LINUX_DEFAULT="nomodeset dummy"
GRUB_CMDLINE_LINUX_DEFAULT=nomodeset

Yast changed the wrong line...