Tenda MX6 and YEELIGHT problem

Hello, I am in Italy and connect to eu servers. I own more than 30 yeelights and all works OK till now. I have bought 3 mesh tenda nova mx6 and they are working greatly with every wireless item on my network (smartphones, tvs, cams, tablets, alexas and google home). Tendas uses the same SSID and pwd than older wifi and older wifi is now offline. 1st generation RGB and B&W yeelight works perfectly, new W3 works, some 2nd generation RGB works but… 6 rgb 2nd generation do not. I tried to reset, use fix ip on dhcp server, use only 2.4ghz network. But these 6 bulbs does not connect to the router anymore. I reactivated an older access point with a different SSID and the 6 bulbs now connect perfectly again. I suppose it is a problem with the bulbs but why only 6 on more than 30 with older and newer models working perfectly?