There it is again.
First of all. Not everyone is deceitful.
The Lord was ministering to Jeremiah, who was being anguished and tormented from watching his beloved people fall into gross depravity of the worst kind. Jeremiah was grieved about how they could do such things.
The Lord told Jeremiah (in the Hebrew) something more like this...
(which makes sense once we know the context).
"The heart {emphasis on old sin nature
wrapped up in mental attitude sins}
is deceitful/fraudulent/'non-reality' above all {things},
and 'dangerously sick' (mental attitude sins lead to temporary or permanent
psychotic conditions).
Who has the ability to discern it?"
Instead, of that understanding? As it should be? Some will run with conventional translations as their means
to condemn all people for being deceitful.
After I graduated from Bible college, I learned the context of Jeremiah by finding a pastor who can exegete and explain from the scriptures the horrible depravity the Jews had fallen into. It was totally gross. While the King James made it almost sound like poetry.
It was why God punished the people by having Nebuchadnezzar come in and over run Israel!
The truth is better than fiction.