Baseball Reference WAR is heavily dependent on park factors for pitcher's which is hurting his value quite a bit. It's why they also had Mike Minor and Lance Lynn as the 2nd and 3rd highest WAR pitchers in the AL last year because Arlington graded as the most hitter friendly park in 2018.Fangraph's WAR grades him out quite a bit better.
Maeda's career PPFp is only 97.3, which is lower than average but not nearly as extreme as Lynn and Minor in 2019 (both in 109-110 range). The Mets starters all had lower PPFp marks in 2019 than Maeda, as did Bumgarner, as did Kershaw, etc.
It might be equal parts park factor and defense which held back Maeda's bWAR compared to fWAR. (And both might be valid questions about Maeda coming over to the Twins.)
Average per season in his career: 1.3 bWAR, 2.4 fWAR. Definitely a difference, although both could be in a similar class of pitcher. (FWIW, those bWAR/fWAR averages, on a per-inning basis, are pretty close to Ricky Nolasco's in the 4 years leading up to his Twins contract...)
Edit to add: some evidence that Maeda is a bigger beneficiary of Dodger Stadium than most, he has the largest home/road ERA differential of any Dodger starter from 2016-2019 (min. 50 innings in each split)