I got a light interrogation as a US citizen. For the record, I have Global Entry, NEXUS, and TSA Pre-Check.
I handed the border agent my US passport and the conversation went like this
"why are you entering the country?"
"I live here"
"do you have legal status in the US?"
"I'm a citizen, you're holding my passport"
"have you ever overstayed a visa in the US in the past?"
"I was born here, so no"
"do you intend to do any work while you're in the US?"
"yes, I'm a US citizen and I have a job"
I didn't get pulled off to the side or anything, it was just standard questioning at entry processing when flying in, but it was just bizarre
the border agent kept looking me up and down suspiciously like I was hiding something, but he had my passport the whole time
even when I got questioned on my way to Canada (I would've stopped me too), they were much nicer about the whole process, it's an air of "we're just double checking cuz making a mistake here would be real bad, but as long as everything's legit, no worries, I hope you have a nice stay in Canada"
entering the US the vibe is "you're a violent criminal and it's my job to ask you questions until you slip up and admit that fact, the US is magnanimous for allowing you to touch our great country's land with your disgusting feet, and you should remember that every day you're here or we'll detain you so you won't forget again"
I'm a little surprised you've only had positive experiences.