Breaking piles of anvils with torches only drops 3 anvils

Precisely what do I mean with this? I was messing around in 1.15.1 by making a little anvil tower. Now, similarly to how you can break sand and gravel by placing torches underneath them, you can break anvils this way. 

So I made a tower of 20 anvils, broke the bottom one and quickly placed a torch, causing all 19 other anvils to break.

After breaking all of them however, only 3 actually dropped.

To make sure it wasn't a one-off occurance, I tried this again with 15, 10, and 5 anvils. Each time, the drop-count capped at 3 anvils.