According to this answer Don't force sync with my oldest account on my network profile states:
Note that this is a one time copy, not an ongoing sync. If you want to update your profile on stackexchange.com after making changes on your Q&A site of choice, you'll need to repeat this process.
However, my profile text (but not my profile image) keeps getting reset.
I have now twice synced my SE profile to my MSE profile and both times as some indeterminate time later it had reset itself back to my SO profile text. I assume it's picking SO as that's my oldest account. I would really like it to stick with my MSE profile.
I have just (2016-12-01 12:30 UTC) had it reset again. This time it was close enough to another event to make me think that they are linked. I'd just created a new account (on Startups to flag some spam) and as it had copied my SO profile I edited the new profile to remove the SO specific stuff. When I refreshed my network profile page (https://stackexchange.com/users/23619/chrisf?tab=accounts) the profile information, but not the profile picture, had been reset. In this case it was reset to being blank which is the "text" I had entered into the profile on Startups.