I haven't reproduced this yet, but someone reported that custom blocks with low mining speed can become invisible when a player tries to break them with an axe. I should check this out someday.

I managed to confirm this bug: it happens when the client thinks it has broken the block before it has received the mining fatigue effect from the server. This can most easily happen when using a diamond axe with efficiency 5 (because it can break mushroom blocks extremely fast).

There is also another problem: axes break mushroom blocks faster, so the effective mining speed can be higher than desired.