When right-clicking with any Pixelmon pickaxe in mainhand, the offhand will not be utilised.
To put the exact example that I discovered this issue with, holding a Ruby Pickaxe in the mainhand (selected hotbar slot) and torches or food in the offhand, if you are looking at a block, you cannot eat or place torches.
This applies for all pickaxes added by the mod. Aluminium, Platinum, Silver, Ruby, Sapphire, Amethyst, Crystal, Fire Stone, Water Stone, Leaf Stone, Thunder Stone (side note, they need to be enchanted from right-click first for other tool types to work, however pickaxe just never works), and Sun Stone.

A similar, but technically different, issue occurs with the Moon, Dawn, and Dusk stone tools, where their potion effects will override the ability to place/eat from the offhand. This one does not necessarily feel like a bug or unintended feature, but rather a Minecraft limitation, so I don't feel it deserves a report itself, but incase the issue in the first part is somehow directly tied to this, I felt it worth mentioning.

Hey,

I was able to confirm that this is an issue, and this will be further investigated. Thank you.

- Swifty

Moved to internal