Maybe it's important to make sure you always add new content to go along with your SEO changes. If the only changes you make are things that Google has tagged as 'SEO-centric' items, I can see how that could be a problem.
Who knows, maybe in their algo, the part that asks "Is there new content?" is higher up in the code. If the answer is yes, then 'let's check that out' - then the if-then closes and doesn't even check for target SEO changes - or looks at SEO changes differently somehow. Just a very uneducated hypo. But I think I will follow that advice since I thought about it. That is if I plan to change SEO items, make sure I add new content along with it.