When running 20w09a (note this has been occurring in all 1.16 early releases) middle clicking on a block while in survival, it doesn't select the block in your inventory. i've confirmed that the controls on the MC client have not been changed, but I I go back to 1.15.2, it works correctly. I've tried this on multiple blocks like cobble, spruce logs, netherrack, diorite, gravel, white bed, spruce trapdoor, spruce slabs, etc.
I've tested having them in my hotbar and also just within my inventory. No difference. I've also tested having an item in my hand or an empty hand. no difference.
There are no datapacks or custom mods on the server. It's vanilla.
Cannot reproduce. Check your controls settings.