I shake mine for a full minute, even though it says 20 seconds or whatever. I think if you let it sit long enough, that might happen, but if you test regularly it shouldn't. Also I'm down to probably less than 10 tests left in one set, so I bought a second set (API) and did a side-by-side test, and the differences were negligible. I shake it like mad and rotate the bottle during.
Out of curiosity though, I think I will be purchasing the Seachem test kit.
This may be moot however, as I have an RO/DI that came with my SW setup that I may start using for all PWCs FW or SW. Mainly on FW to eliminate silicates.