I have almost the same problem but apparently only for hard wired clients. I used to have the problem for wireless as well but it was fixed by upgrading controller to 2.4.11. The clients/managed switches suddenly vanish from the list and appear back in few minutes. At times, the list starts with a few items, and then suddenly grows.

In the logs (under Events), it is showing that my managed switches and some other wired clients disconnect with this message:

5E-3C-22-41-D0-85 was disconnected from network "LAN" on Router(connected time:18m connected, traffic: 0Bytes).

Just to connect back after 4 minutes:

5E-3C-22-41-D0-85 is connected to Router on LAN network.

The funny thing is that while typing this, the machine I am using suddenly appeared in logs as disconnected and disappeared from clients list, however, I'm fully connected and even internet connection works fine.

To me, it is purely a software glitch in Tl-R605. I'm running firmware 1.0.0 Build 20200930 Rel.36519

@MaziarR 

Sorry yes, i should of specified, this is only for LAN connections.. i have loads that randomly appear and dissapear.. seems a prety fundamental role of a router to know whats connected to the network.