I assume you're using LDAP for authentication, not Radius? I know with Radius that the splash page frequency is governed by session-timeout attribute (https://documentation.meraki.com/MR/Splash_Page/Splash_page_frequency) with LDAP, however, I am not 100% sure.
My guess is that it would be governed by the splash page frequency attribute within the Wireless>Splash page. The reason that you had some clients present the splash page after changing this attribute was that they still had the old lease time configured. Therefore now after forcing a re-authentication of the client, they have the new attribute time.
Eliot F | Simplifying IT with Cloud Solutions
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