Item in SNMP checks are always empty after upgrade from 1.6 to 2.0.p17

After upgrading from 1.6 to 2.0.0p17 I found multiple selfmade checks no longer working.
The problem is that in 1.6 the check function is called once for every service it is responsible for having set the item argument as the service that should be checked.
In 2.0 this item parameter is ‘None’ ending in the check function no longer working.
I know that I could convert the whole check to the new api, which is quite a lot of work.
But normally CheckMK 2.0 should be able to work with old plugins and as the plugins having the problems are from a lot of different sources this is not a problem of a specific one (which is the reason I do not include any code example here).
Does anyone know if there is anything I can do to solve this issue without converting the plugin to the new api? Or is there any documentation explaining this behavior? Otherwise I consider this behavior to be a bug.