■ Ah, I figured as much (laugh). So, for item desynthesis, we know the combined maximum is 300 points, and once reaching the max, increasing the skill of one craft will lower the skill of another. Is it possible to, say, lock the skill level of a specific craft to allow you to adjust another?

There's no deliberate locking system but.... I can only ask people try out different ideas.

■ Since it's possible to accidently desynthesize with a class you don't want to level and end up lowering something else...

That won't be happening until you're at the 300 point cap, so for now you can feel free to raise what you like.
So, this is definitely a bug that when experiencing skill points loss before 300 points.