There are no API's in the MKRGSM library to do this currently. The MKRGSM will attempt to connect to any network the SIM knows about, whether it's roaming or not.
Some suggestions:
There is a GSMScanner class that allows you to get the current carrier.
The bottom one has the pads closer together clearly.
Only thing I can think of is stray energy is able to go between pads somehow.
Quality standard for a phone is so much higher that items from the hobby field and that may explain why they both work in a phone.
If it were my card Iwould maybe try score between the tracks a little but that is something for you to decide as it may leave the card inoperable and would require a very delicate hand.
Is there any variation in physical size ?
The top one is also a NANO card punch out type which are the newer ones and always made to a higher spec.
I experiment the same problem !
Is it a problem of the SIM card holder on the MKR GSM board ? Too old to match the contact surface of a nano ? (Surfaces are not exactly in the same design).