Sounds to me like Data-Carrier-Detect might be enabled? If the client disconnects before roaming to other APs, it’s not technically seamless-roaming.
Besides that, I’m sure you’ve already checked the primary details:
- are Both APs are on the same network and tags (ie same SSID)
- splash authentication depends on whether splash is hosted on Meraki cloud or on ISE; if Meraki cloud, use “Sign on” with My Radius.
if using ISE with MAB authentication, then splash must be configured as ISE (vs my RADIUS)