Sounds like *you* have a chip on your shoulder!
You literally said a master's degree is one strike against a candidate and a PhD is 2.
The strikes against these developers aren't *my* doing.
They are literally, purely your choice. The strikes are entirely 100% your doing.
You've got a bunch of largely bogus reasoning about why you have those strikes, but you're still requiring people with masters or PhDs to prove themselves extra hard, which is precisely bias.
The sensible, non-chip-on-your-shoulder attitude would be that for developers, a PhD doesn't make any difference (assuming general dev not some niche specialism), which is something I'd generally agree with.
But that's not your attitude.
So what I have noticed is not due to a prejudice, it's simply a correlation I have observed.
You've built a prejudice based around preconceived notions, selection bias and small sample sizes and now you're in a feedback loop.
Maybe you ought to go back to uni and get a masters in stats or something like that...