Companion reports 18 
Not for me.
I also get 17 on my real phone. But my Memu emulator gets 18.
No, not that emulator, the Memu Play emulator.
MIT Companion, "Do It Result", Installed always 17. Xiaomi.
I have no idea what your problems are. 
I've tried it on multiple devices and Android versions. Result of course always: 17.
Then I live in a parallel universe
Will test later with other phones...
...
sometimes 17, sometimes 18. Even Google does not have a correct answer.
Result number 1:

Result number 2:
Is today week number 16, 17 or 18? I guess we'll never know...
Samsung Galaxy S6 (Android 7):
Same result ...
As I said I live in a parallel universe 
April seems to have an extra week in Greece. 
I am not sure but have some thoughts, does it makes difference to have first day of week as different.

and

PS: I wasn't able to reproduce by changing first day of week in android and window settings.
Look at "Minimal Days in first week", Dora's is 1, Anke's is 4 ???








