3 Way KASA switch turns back on by itself

Hello   @Slocko 

If this is a newly installed switch, please check the Netural Wire is properly wired.

Neutral Wire: usually white or a bundle of white wires secured with a wire nut. If its a bundle of neutral wire, you will need connect the bundle of neutral wires to the white wire of HS210. 

Please also follow the Setup Video  "How to install the Kasa 3-way Light Switch Kit" to and examine the wiring of HS210 step by step. 
https://www.tp-link.com/support/download/hs210-kit/