How can I get grub-probe to recognize an ext4 partition that is recognized by mount?

I tried to install a more recent version of Debian, to a different partition, and I can’t boot into it. The problem shows as error: no such device: a9e0543b-4d58-4729-9da1-ee73f5b148e8 when I try to boot it. The device exists:

$ ls -l /dev/disk/by-uuid/a9e0543b-4d58-4729-9da1-ee73f5b148e8
lrwxrwxrwx 1 root root 10 Jan  7 13:53 /dev/disk/by-uuid/a9e0543b-4d58-4729-9da1-ee73f5b148e8 -> ../../sda7
$

but when I run grub-probe on it, it doesn’t recognise it:

$ sudo grub-probe -d /dev/sda7
grub-probe: error: unknown filesystem.
$

However, I can mount it, as shown here (with my current main partition, /dev/sda5, for comparison) (edited down to relevant lines):

$ sudo mount /dev/sda7 /mnt/os-c/
$ mount
/dev/sda5 on / type ext4 (rw,relatime,errors=remount-ro)
/dev/sda7 on /mnt/os-c type ext4 (rw,relatime)
$

and grub-probe does recognize /dev/sda5:

$ sudo grub-probe -d /dev/sda5
ext2
$

Also, parted recognises it as ext4.

Any suggestions for how to fix this, so I can boot from this partition?