API participant update clobbers participant_fee_level

Updating a participant via the API seems to clobber the participant_fee_level field. Before the update, participant fields look like:

"participant_id":"6517",
"participant_fee_level":["Adult - 2","Child - 1","Service Fee"],
"participant_fee_amount":"57.00",
"participant_fee_currency":"USD",
"event_type":"Performance",
"participant_status_id":"1",
"participant_status":"Registered",

Then I call update via AJAX:

$().crmAPI('Participant', 'update',

{'id':partID, 'participant_status_id':partNewStatus }

, {success: function(data) { .....

And afterward, participant_fee_level looks like this:

"participant_id":"6517",
"participant_fee_level":"Array",
"participant_fee_amount":"57.00",
"participant_fee_currency":"USD",
"event_type":"Performance",
"participant_status_id":"2",
"participant_status":"Attended",