Hello!
I would say that it almost always happens below 40MPH, approaching 2K RPM. And I do notice that when I accelerate, then I ease off the gas, dropping below 2k, reaccelerating that it does produce that sound again. But I’ve been trying to recreate it and get that sound clip for all on the board but for whatever reason, I cannot reproduce the sound anymore. I have no idea if that is a good thing or a bad thing now that it is suddenly “gone”.
It's hard to say, but your experience is consistent with engine lugging providing it only happens when reaccelerating after dropping RPMs. I raise this possibility because myself and some others have experienced this in the 2nd. generation 2.5L non-turbo (including 2017's without cylinder deactivation) when driving in normal mode, not sport mode. I'm surprised it is not reported more often.
The following many words may be excessive for a shot in the dark, but lugging has been my #1 and #1A complaint with the vehicle. I've expounded on this topic several times already but I guess I can't help myself getting into it again. Nitpicking an otherwise fine $26,000 vehicle is not my thing.
So, if you've never driven a manual transmission you might not recognize it as lugging. With a manual, if the driver decelerates, braking or easing off the gas, with the RPM's dropping far enough, and then reaccelerates without downshifting to the appropriate gear, the engine will briefly drone and labor until the RPMs get back up to match the gear. Your automatic may be doing the same thing--failing to drop from 3rd. gear to 2nd. I've also experienced it when the vehicle failed to drop from 2nd. to 1st, such as taking off from a stop sign and then quickly taking my foot off the gas because the guy in front is lollygagging, then reaccelerating with a lug.
It's situational, so you may experience it one day and not the next. It may come and go based on traffic conditions, or the driving maneuvers you need to perform, or how frisky or lollygagging you happen to be that day. If the RPMs are high enough in some zoom-zoom driving you likely won't experience it. You could try simulating a hairpin turn or a U-turn where you brake into the turn, drop the rpms, and reaccelerate out of it. Abandoned mall parking lots, for example, are a good place to try stuff several times.
A 90% cure for me was an update to the current (at the time) version of the powertrain control module (PCM). It now happens less frequently and when it does it's briefer and milder. You didn't say if you bought it from a dealer under Mazda's CPO program. That's uncommon for vehicles with 50K miles but it's possible if it was consistently dealer-serviced. If so, they should have updated the powertrain and transmission control modules to current versions available at that time. In the case of a CPO, if you've concluded lugging is the issue you should take it back to them, report the problem as such, and ask them to confirm for free that the current software versions were installed. If they want to recreate the problem you should be prepared to demonstrate it yourself with a tech in the car otherwise you may get the same answer as with your mechanic.
If not a CPO, then the dealer would have to be exceptionally friendly to do the diagnostic for free at 50k miles. At least one poster reported a $300 charge for the software reflashes though that may vary. If you bought it from a private party you will be out-of-pocket for software updates.
Anyway, if you can recreate the problem and intend to post a recording of the sound, see if you can position the camera at the tach and speedometer.