Was going mining when I discovered this one. My iron pickaxe originally broke, so I crafted a new one with the aluminum I had on hand. I had some torches in my offhand that I was using to light up the cave. Turns out you can't place blocks that are in your offhand while holding the aluminum pickaxe.

Upon further testing in a separate world, I found out this extends to all vanilla tool Pixelmon counterparts, like the Water Stone Pickaxe or the Platinum Shovel. The tools' normal functions (like the shovel's grass paths and the hoe's tilling) still work exactly as expected, but when clicking on a block that won't use these functions like stone, the offhand isn't used. Hammers still function normally (although considering that's to be phased out, I doubt that matters).

Hiya, and thank you for your report! We were aware of this issue already, but I will link your ticket to the existing Internal ticket!

This issue was fixed in 9.0.6. If this continues to be an issue, please open a new ticket.