This is complete BS and I would posit you’re just making things up. Link me to an article about a potential juror spending at least one night in jail due to saying they won’t convict during pre-trial jury selection.
Sure: just search for [voir dire contempt]. The first SERP for me had a law firm page with examples when I looked earlier this evening. I'm not saying it's likely, for what it's worth, but if you lie to avoid jury selection (which is what they're going to think you're doing), it's a possibility.
I got that SERP. The examples are all cases against courts for various violations during _voir dire_. I did not see a single example of a potential juror being jailed for contempt before the trial even started. This isn’t surprising; why would the judge bother holding someone in contempt rather than just moving onto the next potential juror in the pool? It’s not like there is a risk of mistrial from the actions of someone not even participating in it yet.