[Users] Loss of convergence with subcycling in time

[Miguel Zilhão]

>> i’m happy to help dig into this further, but i’m not super familiar
>> with Carpet's internals. should i file a bug report, or is someone
>> already looking into it?
> 
> Sure.

i've just submitted a bug report here: 
https://bitbucket.org/einsteintoolkit/tickets/issues/2920
i added all the information needed to understand and reproduce the 
problem, but please let me know if there's anything missing.

thanks,
Miguel