@ramzinassar 

Connection preference is just a suggestion. It is like at grocery store, where cashier waves their hand to invite you to their checkout lane, but you ignore that and go to different cashier. 

Device does the same on Deco mesh. 

Depending on type of client there may be other reasons. You haven't told what clients ignore connection preference: desktop PC, Smart Thermostat, Smart TV, security camera, etc.

  @Alexandre. 

Thanks. If I disable Mesh for that device will it respect the preferred device?

I understand what you are saying but it's kind of misleading as there is a label in there that says that this device will ONLY connect to that Deco.

  @ramzinassar 

The wording is misleading. I brought it to TP-Link Support attention few months ago, but fixing the wording in the app must have been of low priority.

If you disable "mesh" for device, that device will stop getting Connection Preference recommendations from Deco mesh. If that device already ignores these recommendations, disabling "mesh" for it will not change device behavior.

What you can do, as a workaround, is configure Main Network to run on 5GHz band only:

That band is faster but reaches shorter distances, forcing devices to choose nearby Deco unit.

Connect all devices that support 5GHz to Main Network. For devices that can only run on 2.4GHz, connect them to IoT network and let them be. When time comes to buy a new or replacement device, make sure it supports 5GHz.

Thanks  @Alexandre. 

Main network is already 5Ghz only and I have the IoT network enabled. This is happening for both: 5Ghz devices and 2.4 GHz devices.

I guess I'll have to live with it.