It may be a problem with the software version, mobile phone settings or network lag.

If it is a problem with mobile phone settings, then click Settings, click General on the settings page, click About This Mac, scroll to the bottom, and click Certificate Trust Settings to solve the problem of certificate trust settings.

In addition, take the WeChat Duopen software as an example. The enterprise certificate version of this software cannot be trusted after it is upgraded to iOS15. You can try switching the network, clearing the cache of the browser, and then setting the trust certificate.