I installed openSUSE Leap 15 without problems but NO other installed OS was detected by grub2-mkconfig though I enabled "test foreign OS". There are various different ext4 partitions with Arch, Ubuntu, Fedora and others. I can mount each of them without problems. eg:

mount /dev/mapper/Hitachi_HDT721010SLA360_STF6L7MS281K9K-part1 /mnt
(this partition contains Antergos)

grub.cfg contains only Leap 15 entries, even Tumbleweed was not detected.

Does anyone have an idea?