In our game, I had a Master Builder step forward from a Dwarven settlement I just took (on my mobile phone browser. It was Google Chrome on a Galaxy S6) but I couldn’t see his map card, and when I clicked on his entry from the settlement’s info panel, nothing happened. I then loaded the game in a desktop browser, his map card was there, and I could click on it or his entry in the settlement info panel to get his offer to join me. I then tried again on my phone and was able to see everything and click on everything as expected.
It might be nothing but thought it worth a mention. I’ll respond to this topic if I come across something like this again. I’ll try to repro next time a hero appears to see if a simple refresh of the mobile browser fixes it, or if it required logging in with a desktop browser.