In my experience, I tend to learn more from mistakes than from successes. So that’s why sharing failures is important too 
After doing some post-mortem analysis on the situation, I’m a bit confused. And as the video is not at a great angle, it doesn’t really help to pinpoint where it went wrong. It looks like it went wrong twice.
In the picture cutout A is the last one that was in the right place. For some reason it didn’t go as planned and cuts B & C still worked out, but they were already shifted. Then finally at cut D it went wrong completely.
What I think went wrong:
- not using the right tool. I did use a 3.175mm bit (that has a 3.175mm shaft) but the cutting part of it is only 13mm long. And I was cutting 18mm thick stock. I have gotten away with this before, but it’s always a bit tricky as when you go deep enough the shaft is continuously rubbing against the walls. (and there is no cutting edge anymore)
- Probably in the deeper part of cut A it has gotten stuck and missed some steps resulting in a shifted location. → Problem 1 which can be attributed to using the wrong tool. So a machinist error obviously.
- But then in cut D, the part that came off wasn’t even the cut to full depth. So the tabs that were there held on as expected. at least 3mm of material is still there, it just sheared off completely. Again probably due to the tool not having a cutting edge that’s deep enough for this material, causing vibrations etc.
Possible solutions: get a longer bit or create a bit of a slope (which is possible for this part) so there’s enough clearance. (Or cut half way through, turn the part around and mill from the other side) but in this case that’s too much trouble for the purpose this part needs to serve)
I probably could get away with clearing the pockets completely, but that will take significantly longer and still results in the tool rubbing against the stock from one side, but it won’t be wedged in between anymore probably not causing the same problem anymore. But that doesn’t solve the problem.
Maybe put a slight slope on it and clear out the pockets completely.