Please take a look at the following reference and see if that helps? https://medium.com/k2-dev-en/k2-package-and-deployment-deploy-with-powershell...
Are you using Form Type "Custom"? What version is your K2 Five? Can you attach screenshot of how your Task tab is set up?
What are the steps you used to create SharePoint Online Service Instance?
Do you get the same result if you execute the same method/filter from SmartObject Service Tester tool?
Please also check Permissions required to work with K2 for SharePoint: https://help.k2.com/onlinehelp/k2five/userguide/5.3/default.htm#Welcome/K2A...
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