DirectQuery not working because of certain transformations?

Hi, I am having a problem with a newly created report. In our company we already query data from Harvest through their API endpoint. There it is mostly about projects (500-2000 items). => https://help.getharvest.com/api-v2/

There the DirectQuery works easily within PowerBI.

Now I want to create another report to build some dashboards based on Harvest time entries. Since we have lots of time entries this request is way bigger. But I already filtered the api request and right now I am having 300 entries in my query editor, so not a lot realy. I did some basic transformations (filtered out columns) and then wanted to start creating my dashboard. But now I get a warning that DirectQuery is not possible and it wants mit to switch to import mode.. I have no clue why. I even removed the transformations steps but it does not help.

I would really like to use DirectQuery since the data is changing every day and I want to have the visuals up to date.

So far I wasn't succesful finding the real cause for that problem or a potential solution. Maybe someone can help me out here? 🙂