VSync /update rate (or whatever the real core cause is) related issues are frustrating. Hopefully eventually the engine can be changed to just work in all situations (like the case where VSync has to be manually enabled for the engine on some PCs in graphics control panel / some old iOS devices run the engine too fast in the same way and it impacts things like go/gui.animate animation timings being too quick). I’d guess something similar is getting confused in the situation you’re describing.
You could try setting the frame cap to 60 and unchecking display.vsync under your game.project file and then see how it runs on the mobile Firefox for HTML5 in comparison.