TLDR: None of the data you had before the end of the Ammonia exploration CG will count towards the x3 bonus, confirmed by support.
I had been exploring way out in the black when the CG popped up. I had no plans to return to the bubble but hey, for x3 or x4 bonus on data why not! I spent hours and hours getting back, throwing away plans to finish my trip because hey x3-x4 times my data is crazy and may never come again.
The time finally comes to sell for that sweet, sweet bonus and boom. No bonus. I checked over all the news releases and I was doing everything it asked for the bonus. I then reach out to support, because it is clearly not working, who then inform me that the only data it will apply to is the data I've found in the past day. They also confirmed this was not stated anywhere for CMDR's.
I'm not one to complain in games, but the hours that I put into getting back for this interesting CG to yield nothing AND I'm a bagillion light years away from where I was happily exploring is a kick in the pants.
I hope this acts as a warning to any other explorers returning home for the bonus. Directly from support: There is nothing for you.