@Wayne-TP thanks for your reply and can confirm I have tried a factory reset and the problem still exists.  The lights were working fine for a few months when the problem started.  I have a short video that shows the problem when I ask Alexa to turn off the lights. Please let me know how to send this to you.  

The second set of lights are fixed to an aluminium surround and I have a picture that shows the lights in the off position but it won't let me send in this reply.  Interestingly, when you touch the strip light they become a little brighter but in daylight they are hardly noticeable.  

Good morning, excuse me if I intrude on the same topic, I confirm that I too have the same "problem" with the LED strip that when you turn off the LEDs remain slightly on. I also emailed support directly and they said this is normal behavior. It seems very strange to me that the LED strip must remain slightly on... I'm forced to switch off manually by disconnecting from the power supply... but in this way I lose the smart functionality..

  @EnzoD86 thank you for sharing and your input reassures me I'm not alone with this problem.  There simply is no logic as to why the lights should remain live when turned off and I will not be purchasing anymore as believe it is unsafe.  I have sent my video and photos to them so will be interested to receive feedback on the issue.

@Mushy thank you too, because I had to search a lot on the internet to find someone who had the same problem.
I also think that this way of working is not absolutely correct. I tried to connect a tester to the LED strip and I noticed that a voltage of about 3 volts passes. For this reason the LEDs light up slightly. Support got back to me saying that the strip needs to go into standby in order to receive input from the application. This is also incorrect... because it is the control unit that remains in stand-by (connected to the wifi). Let me know if they answer differently...
As far as I'm concerned, I needed another 4-5 LED strips, but if the problem isn't solved at the software level (with a firmware update, always if possible) I'll look for alternative products.
Thanks, see you soon.

  @EnzoD86 that's very interesting re the voltage test and confirms why the LEDs light up faintly.  I believe others who have purchased will all have the same issue but as they are likely to be placed out of sight (i.e under cupboards, plinths etc) they are probably unaware of the issue.  I have purchased two sets independently and both have the problem,  but my first set now turns bright red when I switch it off which makes it useless as I have to disconnect from the mains.  I have been offered a refund for that set but believe there is a link to the problem we have identified and in time I'm sure others will be sharing their experiences and concerns.  

  @EnzoD86 

  @Mushy I've started to had this issue just a few days ago. I try to turn off the L900 led strip on the fisical button and via the app and it stays always on. I've done at least 3 resets and nothing, always happens. I have another L900 strip on the studio and I've tried with this button on the one that has the issue and it worked fine. I'm assuming the problem is with the button module. TP-Link please let me know what to do. Thank you 

  @njorge they have acknowledged a fault with the lights and offered to replace them with the L920-5 set, however, their returns process is so long winded it prevents you from bothering.  Just out of interest, does the set that is still working remain live when you turn them off?  You can only determine this in darkness by looking at the light strip where you will see a very faint blue glow from the LED's