Unfortunately there isn’t a way to trim a multi instrument’s beginning. You will need to manually destructively edit each playlist item to start at your desired time.
I have added the ability to trim the beginning of multi instruments as a feature suggestion to our tracker.
Just to add to my previous answer - you can work around this with one of the following options:
Create a single instrument with the asset(s) you wish to use in the multi instrument, trim it down to size, right click and choose “Replace with Trimmed Copy”. You can use this trimmed copy in the multi instrument.
Create an event containing a single instrument trimmed to the desired size, then use this event as an event instrument in your multi instrument.