2016~2021 Engine coolant leak

2005 Mazda3
Yes, they actually ended up replacing the entire engine.
Ah gotcha, thanks, sorry I missed it up there, re-read that they were going to swap in that lower mileage motor.

Just an update, after more digging around the entire engine was replaced. Thank God I had that extended warranty as the job would have cost around ~$8000. I was hoping to get a brand new engine, but unfortunately the third party warranty company didn't want to pay for a new one. However the engine they found only had 29,000 miles on it, compared to my 80,000 miles. Also they looked over the engine, replaced all the gaskets and moved my new spark plugs from the old engine to the new one. Not too shabby.
Hope that motor lasts you a good long trouble free time.
Well looks like the same thing just happened to me, as I came across this forum last week as I was looking at possibilities of my coolant leak issue. I have a 2016 Mazda CX-9 Signature and I just hit 60k this week, I started noticing issues about 3 weeks ago when I had an engine hot light come on when I was siting in a parking lot with my air conditioner on. Once the car started to move it went away. When I got home and let the engine cool down I noticed my reservoir for coolant was low, so I added about one gallon. I did not see any issues again for another week or so, but noticed small puddles of coolant under the car from time to time. I called and made an appointment with the dealership and explained my issues. the day before going to my appointment my engine hot light came on a few times when ever I was below 35 MPH, and I was not getting any heat from the left side of my vents for defog, it would come out cold when set at 85 deg. I figured this could be a fan or water pump issue as the bearings might work when at higher rpms. I made it to the dealership with out any issues or breaking down as this had progressed quickly over a two week period. The final verdict was the engine seemed to be cracked and leaking. This was on Saturday morning 9/19/20. Since I just hit 60k miles, and the problem started happening before this happened I am being told it should be covered under the warranty. Also I was told that this is an known issue with my year and model. I was given a loaner for now as they have to complete a full report and submit. I was told they will have to replace the engine. Waiting for further information with my fingers crossed. I will post what the outcome is and the time line to get repaired.
2010 CX-9 GT
That's the first one of these where we got a definitive explanation: cracked block.

Which seems odd, to be honest. I'm wondering if there was a casting issue with a certain batch of blocks or something. Other manufacturers have had porosity problems with certain engines.