Yup! They've been front running you alright, which explains the mystery of why you've been losing money all these years
Let's just entertain the idea that yes, Yahoo is stealing your data. Why not take advantage of it? First buy calls of the stock, then add it to your portfolio, then the stock goes up 1-3%, you profit!! My life advice is, don't get mad, get even (aka take advantage)!
It never ceases to amaze me that people can be sure markets are being manipulated against them in a consistent manner but unable to grasp that they could profit from that manipulation of it actually existed. The paranoid brain goes to great lengths to avoid any disconfirming information.
Step 1: when you add the stock to your list, immediately buy -- don't add earlier. You're effectively guaranteed "1-3%!!". Step 2: decide what name you want for your giant yacht in Monaco.
Enough people following your portfolio and joining you, if you have a reputation, might not be Yahoo, might just be 100's of other users like you maybe ?? does it work that way ??