When blocks are broken while holding that same block in your hand, an empty NBT tag is appended to it.
Confirmed by using an NBT editor/viewer.

Steps to reproduce:

• Install fabric + forgero (latest at time of reporting bug, 0.10.8)
• Create a new world
• Get blocks that can be broken by hand (dirt, sand, torches, etc.)
• Place some blocks down
• While you still have those blocks in your hand, break the one(s) you have placed down

Here is a video showing the bug (uploaded to YouTube as file size was >10MB):
https://youtu.be/zuHXK6xbxXI?t=8

Wow, that is a really wierd issue, but thanks for reporting it! I haven't a clue as to what is causing this issue, as it is far from intended, but I'll look into it. This might also have side effects that are far worse.

Might I ask how you encountered this issue? It seems very odd

I believe I've had this issue as well on my server, can confirm