Native vlan must match which is what you’ve confirmed. Doesn’t have to be vlan 1.
Have you configured vlan 100 in the database on the sg and as @cmr states check your stp
Darren OConnor | doconnor@resalire.co.uk
https://www.linkedin.com/in/darrenoconnor/
I'm not an employee of Cisco/Meraki. My posts are based on Meraki best practice and what has worked for me in the field.