convertToMultiSelect only saves the last selected value

[John W]

We have a simple Behaviour to convert a single select field to a multi-select: 

getFieldByName("Craft").convertToMultiSelect()

When I edit an issue using this Behaviour, I can select multiple values but only the one I choose last is actually saved, and no more than one value is ever saved.

I tried it this way too, but no change:

getFieldByName("Craft").convertToMultiSelect([ 
ajaxOptions: [
query: true, // keep going back to the sever for each keystroke
minQueryLength: 0, 
keyInputPeriod: 500, 
formatResponse: "general", 
]
])

We're running Scriptrunner 6.0.0-p5 on Jira Service Desk 4.6.1.