snow slabs stacked up to the hieght of a half slab do not allow you to step up to a full block.

when you stack snow slabs up to a half block's hieght (4) you can walk up them without jumping. but when you then try to step onto a full block from the slab it won't let you altough the hieght is the same. then how ever if you make the slab 1 snow higher (5) you can walk up that and then from there you can also walk onto a full block
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reporduction
1. Get a 4 high snow stack (not the full block)with a full block next to it and try to get ontop the full block.without jumping
2. Get a 5 high snow stack (not the full block) and a full block and try the same