Manjaro is not the only distribution where GRUB will save the user’s previous selected menuentry
GRUB has 3 options for the default boot option
GNU GRUB Manual 2.04 - Simple-configuration
‘GRUB_DEFAULT’
The default menu entry. This may be a number, in which case it identifies the Nth entry in the generated menu counted from zero, or the title of a menu entry, or the special string ‘saved’.‘GRUB_SAVEDEFAULT’
If this option is set to ‘true’, then, when an entry is selected, save it as a new default entry for use by future runs of GRUB. This is only useful if ‘GRUB_DEFAULT=saved’
If user wants GRUB to default to first menuentry GRUB_SAVEDEFAULT=true should be commented out to #GRUB_SAVEDEFAULT=true (as already suggested)
but GRUB_DEFAULT=Saved should also be changed to GRUB_DEFAULT=0