Some of the bigger dealers know that they're going to get at least one of a high-demand item relatively early on.
Some LGS just put in an order and hope that they get one, so luck factors in as well. I've got an Spec Series V M&P Metal HD (steel frame, limited run) on it's way to an LGS; the shop owner said, 'I don't know when or if I'll get one, so if you see one for a good price, just order it.'
My recommendation is to regularly go to gun.deals or wikiarms, search the SKU numbers (14040 for 686 Plus MG, 14041 for 629 MG) to see if anything is in stock at various dealers, and ideally, not marked up over MSRP.
My understanding with the new MGs is that they're not limited run items. You may just need to wait. The UC guns were getting marked up in February 2024, and were easy to find at discounts 6-7 months later.
Some of it is just lucky timing. I got my 642UC for MSRP shipped shortly after release when I ran a gun.deals search at about midnight on a Friday night. It probably would've sold by Monday afternoon, if not Saturday.
If you're set on the 686 Plus MG, it doesn't look like they've gotten in to people's hands yet. That may change in a week, of course.