As a work around you reliably can view most clients connected via the dhcp table.
The only clients not shown in the DHCP table are the ones who grab a static IP instead of a dhcp lease.
The client map has been slow to fully populate for the last forever (mine always seems to be missing random devices).
I think the reason this happens is because once a device has no more active connections, it is not considered as a active device in the trendmicro device database, even though that device most likely is STILL is actively connected to the network/wifi.
Now get this, THEY ALREADY HAVE IT INTERNALLY FIXED.
If you navigate the firmware, there is different section that queries data from the same connected device database.
AiProtection -> Parental Controls -> Time Scheduling -> Client Name -> Show Offline Client List
They DO have a correctly populated database of ALL clients that were once connected to the network. What they simply have to do is make an assumption and simply implement a timer where once a device has been detected as "active/online", it will remain in the "active/online" list for the next XX hours.
Is it really that crucial to remove a devices online/active status ASAP????
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Of course since this is the trendmicro section of the code, we have no ability to create simple fixes like the on above ....
A lot of the trendmicro code can use improvements / simple fixes across the board. I don't think they care since it really is a MINOR fix.