When you have the silk touch tool at 1 durability, and break a block which would only be obtainable with a silk touch tool, such as cobweb requiring shears ect... They do not give the block you're supposed to get.
For example, the cobweb will drop string when broken with the shears.

Here's a link to a reddit post i made to show a video of it:
https://www.reddit.com/r/Minecraft/comments/dhejc8/just_found_out_that_when_shears_break_they_drop/