Easiest way to troubleshoot those issues would be to take packet captures.
1- Capture on LAN side of the MX Spoke
2- Capture on the Site-To-Site VPN interface of the VPN spoke
3- Capture on the Site-To-Site VPN interface of the VPN Hub
if you see the DHCP requests on all 3 , the problem is not on your Meraki environnement