Yes, I think your are right.
On a clean fresh mailbox, folders structure is ok after connecting on _my_ webmail.
If I connect with the OVH roundcube, logout, and re-connect from _my_ roundcube, sending and delete a mail, I can see this in IMAP log :
[26-Jun-2018 21:10:57 +0200]: <3s3dn349> [38A0] S: * LIST (\HasChildren) "." "INBOX"
[26-Jun-2018 21:10:57 +0200]: <3s3dn349> [38A0] S: * LIST (\HasNoChildren) "." "INBOX.Junk"
[26-Jun-2018 21:10:57 +0200]: <3s3dn349> [38A0] S: * LIST (\HasNoChildren) "." "INBOX.Trash"
[26-Jun-2018 21:10:57 +0200]: <3s3dn349> [38A0] S: * LIST (\HasNoChildren) "." "INBOX.INBOX.Trash"
[26-Jun-2018 21:10:57 +0200]: <3s3dn349> [38A0] S: * LIST (\HasNoChildren) "." "INBOX.INBOX.Sent"
[26-Jun-2018 21:10:57 +0200]: <3s3dn349> [38A0] S: * LIST (\HasNoChildren) "." "INBOX.Sent"
[26-Jun-2018 21:10:57 +0200]: <3s3dn349> [38A0] S: * LIST (\HasNoChildren) "." "INBOX.Drafts"
So I think that OVH roundcube is misconfigured.
My proble is now : how can I change folders subscribing for about 50 users ? Using a mysql file to modify user table values ? But witch ones, an how...