Yeah, I think you're leaving some details out here...
Like when it was built in 1986 it was sold for $1.2M before they bought it at $1.9M 5 years later. Or that your parents listed it for sale $2.6M (2010), $2.2 (2013), $2M (2015), $1.95M->$1.65M (2017)
Or that property taxes decreased over the years, not the land... the "additions" or house. The addition assessment dropped from $1.9M in 2007 to $358k and the taxes went from $38k to $28k despite rates increasing. High was in 2018, $50k tax on assessed value of $2.2M (assessment range was $2.75M - $1.175M)
Based on the fact this house was more or less "on the market" for a 10 year period, and the property tax assessments fell through the floor, this screams house that sat empty and fell into disrepair.
Your "gotcha" post is very atypical.