Probably but I am only guessing as to what could have happened. If it was a BIOS install from the start I would have expected the root to partition to be the one marked as boot.
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Itâs quite an intriguing case this one. During my time here I havenât come across anything like this when it comes to Grub install issues.
Probably, the best approach would be as @Lemon already mentioned above, to boot up the Live ISO in UEFI mode, chroot and install the bootloader into ESP.
Should be set to UEFI only. Itâs the same as the CSM setting being on. It causes problems with installs and booting.
I know that now, but it never caused any problems, until I wanted to update grub. 
I just know from experience with some hardware and a lot of it also depends on the UEFI implementation by the manufacturer. Some are terrible and lacking a lot of settings. What works on some hardware doesnât seem to be the same on others. I know if CSM is left on a lot of times the ISO may get booted in Bios mode or installed in Bios mode. A lot of users either donât know the difference or donât recognize it when it happens. If you have a UEFI system and is dual booting and you install in Bios itâs not going to boot that OS or add it to the boot loader on UEFI. So you would have to use a function key to boot the Bios install by switch boot order. A lot of UEFI now sets automatically secure boot also. My MSI has UEFI only, CSM or i can set UEFI & Legacy. Some other hardware have very little settings. Itâs either UEFI or Legacy etc⌠There is also fast start up and secure boot. Some allow you to remove the secure boot keys also. They are generated automatically usually and on some you can create your own custom keys.
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I also have a MSI mobo and am able to choose between uefi or uefi+legacy.
I canât remember which one I chose.
But it remains odd that I have esp installed and able to boot in legacy mode. 
It is odd as it usually doesnât work.