In the attached example, placing seeds against the side of a snow layer will cause no placement to happen, likely because the seeds are attempting to replace the snow layer but cannot due to the block below said snow layer not being a valid block for planting. In this case it'd be expected the seeds be placed on the farmland instead, as is done when trying to place against a carpet or any other non-replaceable block.

Expected Result

The seeds would be placed on the farmland when trying to place them next to the snow layer