Hi chakie,
I’ve also observed that DHCP clients fail to renew their leases properly. See my recent post for more information: Odd DHCP Client Behavior Using EthernetInterface - Mbed OS / Bugs - Arm Mbed OS support forum.
Based on what I’ve seen and on your post, it does seem to be an issue with one of Mbed’s libraries. Have you had any luck correcting this behavior since your post?
Sorry I couldn’t be of any actual assistance, but I thought it would be useful to get all of this information in one place.