When a site is using Drupal's page cache that's not set to 0 then scheduled alerts fail to display/hide until the cache is manually cleared.
The proper alerts only appear/disappear if you manually clear the Drupal cache or edit an alert.
This is related to the issues addressed in 📌 Scheduling fails with Drupal page cache and cache.page.max_age set to 0 Active .
See the comment added after that issue was closed that describes this issue: https://www.drupal.org/project/sitewide_alert/issues/3189144#comment-161... 📌 Scheduling fails with Drupal page cache and cache.page.max_age set to 0 Active -- It calls out 🐛 The Sidewide Alerts load route set a maximum cache max age of 30 in an interesting spot Active as the cause of this and I believe that assessment is correct.
I suggest more-or-less reverting the change made in 🐛 The Sidewide Alerts load route set a maximum cache max age of 30 in an interesting spot Active as that fixes the problem. I was tempted to just use the `cache_max_age` setting directly here. I'm honestly not 100% sure if it's what we want to do. I've erred on the side of keeping fresher data at the origin and am using 30 seconds, or cache_max_age, whichever is lower.