RADIUS for splash is something different. It's when you want a splash page and want it to auth users via RADIUS.
Standard RADIUS doesn't use splash. The order of the RADIUS servers is tried from the top down. The advanced fallback feature allows for the APs to rollback to the higher priority servers when they're reachable. Even without that feature the list is tried top down.
To validate this as a test I configured bogus RADIUS IPs for my #1 and # 2 servers in the list. My clients still auth successfully because the AP tries the first two and fail as expected and it succeeds on server #3. I also tested in an alternate way by having valid RADIUS IPs, but blocking the first and second via a firewall rule.
If the list of multiple servers didn't function a list would be pointless to have in the first place. My recommendation is circle back with Support as it sounds like maybe there's a misunderstanding here.
Here's a snippet from the firewall log showing what occurs when my clients auth. They first try the bogus RADIUS server I have listed as #1 (1.1.1.1). That of course fails and it moves on to the #2 RADIUS server IP which is valid (JumpCloud) and my clients successfully connect. This not relying on RADIUS fallback. This is simply the default behavior of trying alternate RADIUS servers when the first one fails.
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