Is that an OEM seal or the weird ones that a batch or two of Boundary pumps shipped with?

It came with the inferior ones, but I installed an OEM one 2 months ago. I remember it was pretty easy to push it in. I have just replaced it with another OEM seal, but this time I put a little coat of RTV on the outside. Still, it went in pretty easily. I didn't have to use a hammer or anything, just a little push with my fingers. I guess that's not helping with keeping the seal in.

the more important question is, what is causing the seal to pop out? I have a freshly built engine, a pretty good crank case ventilation (tiny hole mod, copper scrubbers, -10AN lines & catch can for cruise, breather filter for boost) and no oil neither in my catch can nor inside the breather filter?!

That seal should not just push in easily. Mine you have to push about as hard as you can with your thumbs, or hammer them in. When I do mine, I clean the bore till it's spotless and oil free, apply thin film of RTV to bore and outside of seal, then install seal. I use OEM seals.

That's what I did: degrease throughly, apply RTV, press OEM seal in. Still it went in pretty easily - both times!

That is strange, I've never had that problem with a stock pump, it's always a pain trying to get that seal to go in. Has the bore for that seal been damaged or something? Or bored out? If it keeps falling out and you're doing all that, my guess is the bore is too big for some reason.

Should require work to install. Something is amiss.

Nope, I bought the pump last winter brand new. Cost me a small fortune with all the import duties and VAT. This is the second original Mazda seal that I can push in by hand. We'll see whether the RTV fixes the problem.