We are having this problem in Chrome as well, and it all started with Chrome update 44.
I tried using "\t", :tab, :return, :enter, and :escape to remove focus from a text input (symbols found here), but none of them are working in the most recent version of Chrome.
I know you mentioned you are not looking for solutions other than tab, but short of rolling back your browser version and disabling updates, or waiting for the possibility of a fix from Google itself, you will have to change the scripts being executed.
We found that this has worked well so far:
driver.execute_script("document.activeElement.blur()")
If the desired behavior in your case is to shift focus two fields forward, then you might consider another Javascript solution that looks something like what is discussed in this thread:
simulate the tab key function in javascript
Of course, this probably isn't the best long term solution, as even some very quick Googling shows that sending a tab key to switch focus has been recommended tons of times to people using WebDriver. Because of this, I'm keeping my fingers crossed there will be another Chrome update to revert the behavior.