@yonz 

It is possible your Deco B is too far from Main Deco to establish good link to it. Either that, or some sort of WiFi interference happens occasionally, such as microwave gets turned on or cordless phone is in use - which weakens Deco B link to Main Deco to the point of Deco B deciding to reconnect to Deco A. 

Check what is the quality of the link between Deco B and Main Deco, when Deco B is connected to Main Deco. Also, when Deco B is connected to Deco A, check the quality of the link between them, too.

Also, try moving Deco B closer to Main Deco.

I don't think that's the case - when I'm turning off A, everything works great in Main's and B's areas.

Even if what you're describing is correct, what's the point of the settings I applied in the deco's app if they are just getting overwritten?

  @yonz 

Let me start with saying that your case is unusual. Deco mesh prefers star topology. People complain on that forum that Satellites connect to Main Deco even when WiFi backhaul between them is weak. TP-Link even posted an article explaining rationale for star topology:  https://community.tp-link.com/en/home/forum/topic/276958

In your case, Deco mesh daisy chains on its own and you don't want that. You also can't force star topology to stay, through Deco app configuration settings.

The setting in Deco app you use is called "selecting a preferred signal source." Note word preferred, the point is to emphasize this setting will be used as long as Deco mesh can. When Deco mesh can't, Deco mesh will ignore the setting.

For example, suppose you configured Satellite Deco 2 connect to Satellite Deco 1. Then, you power off Satellite Deco 1, and backhaul signal between 1 and 2 is lost. Deco 2 will either have to stay disconnected from the mesh, or to find another Deco to connect to. In Deco mesh connectivity algorithm, Satellite Deco 2 will try to find another Deco to connect to, overriding the preferred signal source setting. When you power back on Deco 1, Deco 2 will either reconnect to it soon, or after Deco 2 is bounced. 

Based on that, the only logical conclusion in your case, other than firmware bug nobody else reported yet on that forum, would be Deco B can't keep WiFi backhaul to Main Deco. As I suggested, run the test: move Deco B closer to Main Deco. Find new place for Deco B approximately half way between Main Deco and where Deco B currently is. If that makes Deco B stay connected to Main Deco, and connection is stable, it will support my original theory.