I've noticed this as well, particularly as a v0 to v1 problem.
The documentation for v0 actually states to use a comma-separated string for fields, which works for v0.
https://developer.cisco.com/meraki/api/#!get-network-sm-devices
This fails in v1. Trying to use an array doesn't work either.
This works (v0):
https://api.meraki.com/api/v0/networks/{{networkId}}/sm/devices?fields=id,name,lastUser,ownerEmail,ownerUsername
These do not (v1):
https://api.meraki.com/api/v1/networks/{{networkId}}/sm/devices?fields=id,name,lastUser,ownerEmail,ownerUsername
https://api.meraki.com/api/v1/networks/{{networkId}}/sm/devices?fields=[id,name,lastUser,ownerEmail,ownerUsername]
https://api.meraki.com/api/v1/networks/{{networkId}}/sm/devices?fields=['id','name','lastUser','ownerEmail','ownerUsername']