I've been on the fence about whether to get MathStudio Express for a while now. I've been watching the reviews and the bug reports with keen interest, trying to decide whether it was a sufficient upgrade over the previous MathStudio I already had to warrant purchase, but I saw an opportunity to be helpful here, and so I have bought MathStudio Express specifically to try to help you track down this bug.
You see, unlike many here, my device is an official Google release, specifically the 2013 HD (1920x1200) version of the Nexus 7 tablet, running Android 6.0.1, on which I've just run this ImplicitPlot test, and it is displaying the exact same errant behavior as tjdgqx is experiencing.
The reason this is important is that I believe Android Studio, which I presume you are using for your Android software development work, can emulate any of the standard Google devices just by picking it off a menu. I therefore suggest that you try your current development build on an emulator of the 2013 (2nd Generation) Nexus 7. If further details would be helpful, it is the WiFi-Only version, running the January 1st, 2016 patch level, build number MMB290. It has not been rooted, nor is it running any variant OS such as CyanogenMod. It's just the stock firmware and stock OS, with various apps installed.
If the problem does not manifest on your emulator, then there is something wrong with Google's emulation mechanics. If it does manifest on your emulator, then it should make it much easier for you to test changes and track down the cause.
For more details on the behavior I am witnessing, when I enter the command ImplicitPlot(sin(x)=y), it comes up in "Positive" mode, and the graph is visible. If I tap "Positive", it changes to "Negative", and the graph disappears completely. If I then tap "Negative", it switches to "Lines", and the graph is still invisible. If I then tap "Lines", it switches back to "Positive", and the graph becomes visible once again. Neither the "Use Smooth Lines" nor the "Use Smooth Shading" options seem to have any effect. The grid lines remain visible the entire time. This, we will call Bug #1.
Bug #2 happens if I then tap the Home icon to pop out to the desktop, and then switch back to MathStudio Express. This causes the ImplicitPlot to enter a state where everything is invisible, including the grid lines. If I then double tap on the ImplicitPlot to go full-screen, the grid lines may or may not return. If they do, then when I double-tap it again to leave full-screen, the grid lines will persist and we are back to the behavior described in the preceding paragraph.
Possible Bug #3: If I repeatedly double-tap on the plot, to keep switching in and out full-screen, the x and y ranges of the graph keep expanding, increasing by a factor of the square root of 2 each time. So if they start at +5 and -5, they will be around +20 and -20 after four cycles. It seems to update the x-min, x-max, y-min, and y-max values on the first tap of each double-tap used to enter full-screen. Is this behavior intentional?
FYI, I am a programmer and a fairly skilled debugger in another domain, but I do not code for Android, so I may or may not be able to be of further assistance. I await your response with much curiosity.