I verified this issue. First I change the /etc/grub.d/20_memtest86+ like below.
-menuentry "Memory test (memtest86+)" { +menuentry "Memory test (memtest86+)" --hotkey f9{
After update-grub and reboot, F9 could work.
Next, I change the /etc/default/grub, I added 'GRUB_TIMEOUT_STYLE=hidden' After update-grub and reboot, F9 could work.
Next, I change the /etc/default/grub like below. -GRUB_HIDDEN_TIMEOUT=0 GRUB_HIDDEN_TIMEOUT=10 After update-grub and reboot, F9 could work.
So, I thought this issue should be fixed.
I verified this issue. d/20_memtest86+ like below.
First I change the /etc/grub.
-menuentry "Memory test (memtest86+)" {
+menuentry "Memory test (memtest86+)" --hotkey f9{
After update-grub and reboot, F9 could work.
Next, I change the /etc/default/grub, I added 'GRUB_TIMEOUT_ STYLE=hidden'
After update-grub and reboot, F9 could work.
Next, I change the /etc/default/grub like below. TIMEOUT= 0 TIMEOUT= 10
-GRUB_HIDDEN_
GRUB_HIDDEN_
After update-grub and reboot, F9 could work.
So, I thought this issue should be fixed.