Yes, we have the similar issues with all mobile devices - especially since we have connected the FortiAPs (U321E) directly to the FortiGate (before we used a FortiWLC device, which isn´t supported by Fortinet anymore).
Actually we haven´t any solution for this, but my coworkers tell me, that the problem is really annoying, because the devices often don´t signalize calls (Teams) or other office apps don't get recent changes (i. e. email, teams chats).
On the one hand, my employees (=customers) are not satisfied and on my side, the many requests and complaints tie up my time and mind...
Maybe it´s time to search for another working solution from another producer?
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.