As long as it sinks, it shouldn't matter, since the Hanna photo eyes are about 2/3 the way UP the chamber.
I wasn't sure if these magnetic pill stirrers were good for powder reagent but now I'm interested.
I've always assumed the shake 'gently' instructions are for the ones that will create lots of bubbles and/or hold those bubbles in the solution for more than a minute or two. Shaking gently should help keep them from forming in the first place.
Alternatively, they might prefer they're all shaken gently to keep the bubbles out, but the ones that need to be shaken vigorously are the ones that the powder doesn't easily dissolve into the water.