I have a 2017 Hai cross trek and 2018 Hai TREKING the seat post prevents the seat from going all the way down to the frame. Has anyone cut their seat post? How much of the seat post needs to go in the frame?
If it's truly down that far, and assuming you don't have it down too far at an unhealthy level for your knees, then by all means lop some off.
Look at the end...there is a minimum safety insertion length marked there, something you won't have to worry about. Cut less...you can always cut more later.
I had a similar experience and bought a shorter seatpost - they're not that expensive. All Haibikes come with 31.6mm diameter seatposts;most are 350mm long (I'd wager yours are but you can measure them to be sure).
Instead of cutting, you could buy a 300mm post, which would have a proper "minimum insertion" mark to prevent potential damage to the seat tube.
My Haibike Trekking RC does not have a straight seat tube, so the bottom of the 400mm seatpost stops at the bend in the tube. The seatpost head is still 5-6" above the seat tube collar, so it is definitely not too big a bike. This would be the issue the OP is having. As LimboJim points out, a shorter seatpost could be used. As for how much seatpost needs to be inserted into the frame, this is the Consumer Product Safety Commission requirement: "The seat post shall contain a permanent mark or ring that clearly indicates the minimum insertion depth (maximum seat-height adjustment); the mark shall not affect the structural integrity of the seat post. This mark shall be located no less than two seat-post diameters from the lowest point on the post shaft, and the post strength shall be maintained for at least a length of one shaft diameter below the mark."
So with a seatpost diameter of 31.6 mm, you should have at least 64 mm of post into the tube.