Hi @wellisi,
I have made a test on my side and have created two lists as below:
The AssignTo column is a Person or Group column allow multiple seletions.
The SingleAssignTo column is a Person or Group column don't allow multiple seletions.
Then I have created a flow as below:

The expression of SingleAssignTo Claims as below:
concat('i:0#.f|membership|',items('Apply_to_each')?['Email'])
When an item is created in the List A as below:

The flow would run successfully as below:
The items would be created in the List B as below:

Best regards,
Community Support Team _ Alice Zhang
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