After further investigation in the memtest86+ package, I discovered the bug affecting me has already been fixed.
The update-grub command run during my upgrade process was done by the old package after it was removed (by the postrm file). The new memtest86+ package doesn't run update-grub any more.
So, this bug seems fixed for future upgrades. But any other user upgrading to 8.10 with memtest86+ and do_bootloader=no will encounter the same problem and will have a unbootable system until he runs update-grub manually.
After further investigation in the memtest86+ package, I discovered the bug affecting me has already been fixed.
The update-grub command run during my upgrade process was done by the old package after it was removed (by the postrm file). The new memtest86+ package doesn't run update-grub any more.
See also http:// bugs.debian. org/cgi- bin/bugreport. cgi?bug= 460080
So, this bug seems fixed for future upgrades. But any other user upgrading to 8.10 with memtest86+ and do_bootloader=no will encounter the same problem and will have a unbootable system until he runs update-grub manually.