I've attached 2 files showing the result with and without trimming enabled (two 64x64 tiles, one placed above the other). In case you want to investigate you can find a tiny unity project where the problem occurs for me located right here: link removed - only post links in the private support forum
It works perfectly when one disables trimming, the problem occurs when trimming is enabled. (The tilesheet is all yellow because I am not at liberty to share the artwork in use, but the problem is not exactly less visible with this bogus artwork)