Our website uses an open source product to detect user agent and based on that, it servers mobile theme or desktop theme to users. I know the exact difference between mobile-friendly test and pagespeed insights as described here https://developers.google.com/webmasters/mobile-sites/references/faq#whats-the-difference-between-the-pagespeed-insights-ux-score-and-the-mobile-friendly-test-results
So if a MFT uses a normal Google bot to request the page, then of course desktop theme will be served. But where more appropriately PageSpeed Insights uses mobile user agent to request the page. So it will be served with mobile theme.
Page speed result gives 100% pass on mobile usability and in mobile devices, it loads mobile theme correctly. But on other-hand result in following link says 0% pass on mobile usability.
So now we are getting many pages marked as not mobile friendly even-though we have a mobile friendly theme.
In last few weeks when i click Mobile Usability Check link from my web master account, it loaded PageSpeed. From today its loading MFT.
I believe fetching page as normal Google bot and indexing for mobile website will have issues. I am sure there may be 1000's of websites with 2 different themes for mobile and desktop users.
My question is: anyone else faced same situation and found a solution or found a reason why its behaving?
Additional Information and Findings:
My application is based on .Net and i have disabled all the other scripting PHP, Perl etc on my production Server except .Net in IIS Server. I have got some more clue on this issue as well. We are using browserCaps.config and 51degrees to detect mobile devices and webcrawlers. So some where, detection is causing issue with Google mobile crawler. Treating mobile crawler as desktop browser.