Just writing this for my own posterity. Maybe its already a commonly known tidbit.
If you want to filter on datetime fields from D365FO OData services, from PowerBI, you have to do it in a specific way. What made it work for me was the following.
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						let     PO_Lines = OData.Feed("https://afahrenholz-dev24d2d448f42b83303devaos.cloudax.dynamics.com/data/PurchaseOrderLinesV2?"     & "$select=dataAreaId,PurchaseOrderNumber,LineAmount,PurchasePrice,ProjectId,ProjectCategoryId,ProcurementProductCategoryName,VendorInvoiceMatchingPolicy,RequestedDeliveryDate"     & "&$cross-company=true"     & "&$filter=RequestedDeliveryDate ge " & Date.ToText(Date.AddDays(Date.From(DateTime.FixedLocalNow()), -PeriodBackInTime), "yyyy-MM-dd")   //& "&$filter=RequestedDeliveryDate ge 2021-01-01"     , null, [Implementation="2.0"]) in     PO_Lines   | 
					
Make sure to put the $filter after the $select, and make sure there are no line-breaks in the URL string
Cheers