I seems to be confused as to if it's a multi picker or not...

simply resaving in the datatype, corrects it.

exportingin a package prior to any alteration, the data matches what exports once it been resaved.

xml
<DataType Name="MY - Media Picker" Id="Umbraco.MediaPicker3" Definition="d195af77-4fe1-41ed-a70e-91cad8f35596" DatabaseType="Ntext" Configuration="{"filter":"Image,umbracoMediaVectorGraphics,umbracoMediaVideo","multiple":false,"validationLimit":{},"enableLocalFocalPoint":true,"crops":[],"ignoreUserStartNodes":false}" Folders="MY+-+Content" FolderKeys="dce161a3-cee7-4fe0-be04-177042691ac6"/>