Token for Contribution custom field outputting "Array" for a Multi-Select custom field
I found I could get the value I needed by using contribution.custom_xx[0], though I was dealing with a field with only one option.

For multi-select I presume you would need a foreach to output all of the array values, or implode it into a string if you just want the contents as a list.  I had tried implode but found I didn't need it, something like this might work
{', '|implode:contribution.custom_xx}