Comment 3 for bug 1864677

Tests showed that this problem (grub install) does not happen with the live-server image, which is the default image for 20.04 and includes a new installer (subiquity instead of d-i - we now started to flag d-i as legacy).
The image is available here: http://cdimage.ubuntu.com/ubuntu/releases/20.04/release/
If you are prompted to update the installer, when you started the installation at the 2nd screen (after you selected the language), it's recommended to do so to benefit from the latest version with latest fixes.
I'm closing this ticket now - in case you will run into other problems, please open a separate LP bug. Thx