Comment 21 for bug 2060624

I finally found the cause, the two big hints were:
- 2 fresh installs were in this order sda1, sda2
- only happens with install *without a reformat to sda1*

After the first install to sda1:
- MBR (and MBR gap) contains GRUB core pointing to sda1, sda1 contains modules and config, this config on sda1 contains no os-prober entries as this was a fresh install

After the second install to sda2:
- MBR (and MBR gap) contains GRUB core pointing to sda2, sda2 contains modules and config, this config on sda2 now has os-prober entries for the install on sda1
- everything seems normal
- sda1's grub config still doesnt contain os-prober entries, but it is dormant now

After the third install *without reformat to sda1*:
- MBR contains GRUB core again pointing to sda1, sda1 contains modules and config
- while the installer regenerates the GRUB config here, during this re-generation the old config *on sda1* (due to no format) is checked for the presence of os-prober entries, and these won't appear in the dormant config from step 1, thus no new os-prober entries for the install on sda2 will be generated

As a result sda1 was just a symptom and not the true cause, the true cause is overwriting an install with a dormant grub config containing no os-prober entries.

The reason a no reformat re-install to sda2 didn't trigger is due to the existing grub config containing os-prober entries from step2. If the initial installs were in a different order, e.g. sda2 then after it sda1, then installing without reformat to sda2 will also trigger it.