Exactly. That's a good thing.
A jury is supposed to be representative of the public. Biases (in any direction) are impossible to eliminate; Rejecting jurors based on their answers to questions only makes the problem worse.
So you're ok with someone walking because a racist on a jury refuses to convict someone of that race?
Or they're determined to convict because it's their ex- on trial?
No one juror can convict a defendant. In my jurisdiction, a verdict must be unanimous. It takes all twelve jurors to convict or to acquit. And jurors are required to declare to the judge if they have personal knowledge of the defendant, or even of the crime scene - at which point they are usually replaced (or if they fail to declare their prior experience, are in serious trouble if caught out). A defendant's ex would be recognized in court by the defendant (if by nobody else), and would be very unlikely to get away with failing to declare an interest.
So no, I am not "OK with" your two ridiculous ad-hoc scenarios, and nor are they remotely plausible.