Background:    

Some users might notice that all satellite Deco units are connected to the main Deco, instead of the nearest satellite Deco.

Typical Layout:

Why does the satellite Deco still keep selecting the main Deco as a signal source instead of the nearest satellite Deco?
Deco will choose the best signal source by itself. The selection does not depend absolutely on the signal strength, but also on the connection rate.  Sometimes satellite Deco units may get slower speed due to half-duplex mode and interferences caused by connected devices like phones, or other devices.  To ensure the satellite Deco gets reliable speed, the main Deco stays selected by Satellite Deco after comparative analysis. 

Some users tested that after moving one of the satellites far enough and not being able to reach the main Deco, they finally get the desired connection map:

However, the speed of the satellite becomes slower than what it could get when connecting to the main Deco directly.  Due to the working pattern and protocol/regulation of Wi-Fi technology, the more wireless hops, the slower the speed would be. This rule applies to any kind of mesh or extender system. So it is not always good to connect to the nearest Deco unit.

Solutions
TP-Link has planned to give an opportunity for our user to manually select the source Deco as well as improving the current selection mechanism, to better adapt to the various user environments. You may keep an eye on firmware upgrades.

Before the official release of this firmware, the current workaround would be:
For Deco M5, there has been a firmware 1.6.0 for trial:

[New Firmware] Deco M5 1.6.0
For other models, please install the following firmware to monitor if the satellite will connect to the nearest node:

Deco M9 Plus V1 1.5.0 build 20210205

Deco M9 Plus V2 1.5.0 build 20210205

Deco X60 V1 1.5.0 build 20210205

Deco X60 V2 1.5.0 build 20210205

Note: Please be sure you have read The Beta Test Agreement before proceeding!

Related FAQ:

Is the Signal Source for Satellite Decos Selectable on the Deco App?

How can I find a suitable spot for my Deco?

Why is my satellite Deco unit not getting the same speed as the main Deco?