Hey bro, I have run into the same thing while working on my project Hypic. The Deco PX50 setup keeps switching from a strong PLC connection to weak Wi-Fi for no reason. Rebooting helps for a bit, but it always slips back. Hopefully TP Link drops a fix soon, ‘cause it really messes with smooth project work.

  @Zuckyliessss 

thanks for your feedback confirming the issue.

In the mean time i saw in this forum that the exact same problem is reported by multiple users since 2023....   so I'm not convinced that TP link is in control....

  @David-TP 

Hi David

• As you suggested, i powered down jardin by shutting down the POE/Ethernet  link between cellier and jardin.  In this case the powerline link between cellier and grange goes up and remains stable and the powerline signal is still reported as strong by grange for several hours.
• I created a case via the app as you suggested, I included the description and screenshots.
   how can I update the case with the logs takien from the DECO itself ?

  @David-TP

This problem should not be too complex to reproduce in TP-Link lab, you need 2 DECO PX50 and 2 DECOX50-OUTDOOR and interconnect them as described in the problem statement/drawing. 
after some time  the powerline link should be ignored and replaced by the ETHERNET-WIFI-ETHERNET path.

IMO a powerlink between 2 devices should always be preferred over  any other path involving a WIFI link, whatever WIFI signal strength

Thanks and BRs