Firstly, thanks for yours great job in develop a great software.
After that, I faced some trouble with using lwip2 for esp8266 (I am not really using it, I work with arduino framework), if i compile my project with lwip-1.4.1 it can work with some kind of wifi AP, and some others it is less stable. If i using lwip2 for my project, may be it is more stable in keep connection to the internet. My problem is some APs, lwip-1.4.1 can work and have internet but lwip2 completely have no internet connection. I send the Ping package to 18.104.22.168 with lwip-1.4.1 package size is 60 and receive package is also 60 bytes (and from 22.214.171.124 sent to me), but lwip2 sending package is 60bytes receive package is 56bytes (from default gateway, not 126.96.36.199), at that moment other device connected to that AP still work with internet and the esp8266 with lwip-1.4.1 also have internet.