After every kernel-update, Grub scans the disk for multiple operatingsystems.
If it finds more than one, it enables the menu.
Maybe Grub is not configured to scan more than one harddisk and doesn't enable the menu because your operatingsystems reside on different harddisks, I don't know, never encountered that.
But in this case you can always use the above described manual override to always show the menu.
That is exactly what happens. BIOS -> GRUB -> Ubuntu-MATE.
The only thing is that GRUB hides its output so it seems like it isn't there.
After a kernel update seems to be the most logical conclusion.
By the way, I also have an X570 board ("MSI mpg X570 gaming plus" with a Ryzen 5 3600 ) but I couldn't tell if that plays any role in this. I have a different dual boot configuration. I have a tiny Win10 partition on my Ubuntu NVMe. Just enough OS for playing games that are stored on another disk.
Cool, let me know if it worked for you, okay ? 