[yocto] passwd fails for other users than root

[Arno Steffens]

I have 2 extra users (added with useradd_example.bb). Login works for all 3 parties.

While login works, passwd (to change it) doesn't work for the extra users. They will asked for the old one - which fails.
Root is not to be asked for old password. That's way root can change password for all. 

The strange issue is, that after root changes password for users, they can do it by themselves.
Seems that login and passwd use different algos!?!?

Same password, but different hashes:
before (created by yocto)
user:$6$bk.Az/8KVV$v3TyAlroQeBZA3faJ.DxCVsCUBZaSAvB4FkuNkLDdsIchrmz6OnUGRnQI5BOgOjQ3mvj9QL3US6Amf2VWr0.j/:17598:0:99999:7:::
behind (changed by root):
user:$1$d3Uo1g82$Il2kYQj8qadzQvkwUU4Ia.:17598:0:99999:7:::

Edit: After a few further test I found that this has an effect:
I used EXTRA_IMAGE_FEATURES += "read-only-rootfs" to create a read-only-rootf (by default). 
But of course I did make the partition writeable before attempting to change the password.
But for some strange reasons this is not same. What might be the reason?