Currently, the 'in' operator could not be delegable within SP List data source, I think this is the issue where you faced the Delegation warning issue.
If the amount of your Sublist records is more than 2000, you could consider bulk-load the records of your Sublist into your collection, then use the collection as data source in your app instead of your original Subliost data source.
Please check and see if the following alternative solutions I mentioned would help in your scenario:
Please consider take a try with above solution, then check if the issue is solved. If you have solved your problem, please consider go ahead to click "Accept as Solution" to identify this thread has been solved.
Community Support Team _ Kris Dai If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.