That is a coolant leak, there is nothing else that leaves a frost pattern like that. The reason the frost pattern does not go throughout the entire coilset is because there is not enough coolant to frost the entire coilset as the compressor pushes it through. It will get somewhat cool but after that amount of time those coils should have been frosted from top to bottom.
That means you will have to call for a coolant leak repair and the following questions are the ones that you should ask before accepting an agreement for an appliance
Do you have a license to recover freon? (They must be licensed its federal law) That way they just cannot come out without you knowing and charge you then leave because they do not have one.
What is the minimum you charge for a sealed system leak repair?
Please and lastly ask them to bring the tools needed to repair a leak. They may find the part of your unit that is leaking and be able to repair it that day, that could save a second trip charge. What some will do is tell you they have to bring the tools showing up without them and then charge twice for coming to your home twice, you want to try and avoid that.
This will give you some idea if you want to repair it or not.
If you have any questions at all about anything please ask me. If not then please leave a rating for me, a single rating is very important.