I can confirm this bug still exists in debian-13.3.0-amd-netinst.iso.
However, I have seen older installation videos using debian-13.x (unknown major revision) using the graphical installer which did not show this behavior. It might be this bug is only in the text based installer.
The workaround is to leave the partitioner and perform a manual format in FAT on the EFI partition and return to the installation process. I am not sure if this is possible by entering the Ash shell provided by the installer.
I just aborted the installation, booted from a live USB, formatted the EFI partition and restarted the installer. This enabled the mounting during the installation process and further installation went smooth.
jlinkels
