Maine has one of the largest proportions of D-leaning independents of any state. This poll shows Mills has retained their support, and this race remains Lean D provided Mills keeps them in her column.
Remember that recent polling errors of big magnitude in this state have occurred with Donald Trump and/or Susan Collins on the ballot. In 2018, polls were pretty close to the actual results of the Senate, governor, and 2nd district House elections. Without Trump and/or Collins on the ballot in 2022, I would expect polling errors to be much smaller than they were in 2014/2016/2020.
By the same token this poll shows LePage faring better with younger voters than older (which would never happen on election day) so the possibility of systemic response bias cannot be ruled out.
