So yeah, adding words does add subtext, but your phrasing of this seems to imply that there is existing subtext. What do you see, specifically?
"One in 5 people now think" is a positive-sounding construction which lends more statstical weight to the result than an impartial examination of the numbers, and people who scan headlines are far from impartial.
Other constructions are "4 in 5 still support...", or "80% still support", or "20% now think", which are all still mathematically neutral, but the subtext is different.
I'm struggling to figure out a neutral phrasing involving this data point. My reading of your response indicates that you see a temporal indicator ("now") or a delta indicator ("still" / "no longer") as both introducing bias. "One in five people think Brexit was the wrong decision" without anything clarifying significantly misrepresents the data.
It seems your issue is actually with the data point being in the headline at all. Maybe the headline should be a dry thing like "A Brexit survey happened, read inside for details". I won't entirely disagree. But I think this is less about word choice than you think.
I think that using "20% now think that..." would give a more accurate impression to someone scanning the headline.