1.15.2, 1.16.1, 1.16.3, 20w49a, 21w03a, 1.16.5, 21w18a, 1.17 Pre-release 4, 1.17.1, 1.20 Pre-release 4
Several blocks require a pickaxe specifically to drop anything, and the best tool in the game of any other type will break them very slowly and yield nothing. I would expect this to be the case even with silk touch tools.
However, when mining an infested stone block with a silk touch tool that is not a pickaxe, it still 1) breaks instantly and 2) drops the regular version of that stone block.
1. Enchant a shovel (or axe, etc.) with silk touch.
2. Use it on an infested stone block.
3. The block breaks instantly and also drops regular stone.
Unknowns / Potential Limitations:
I have not tested this with infested versions of cobblestone or any stone bricks. The specific conditions under which I found this issue were with using a max-enchanted, named, diamond shovel in Peaceful Survival, and then replicated with a diamond shovel enchanted with only silk touch in a creative world switched over to Peaceful Survival.
Other blocks do not appear to show this issue, as I used the same test world and when attempting to mine 1) coal ore and 2) emerald ore, the silk touch diamond shovel still took several seconds to break the ore and nothing was dropped.