Welcome.
That's an odd one too.
If I where to guess, maybe the fuel pump screen is clogging up and or the fuel pump is on its way out. Or one or more fuel injectors are clogged or a bit blocked.
Another guess, maybe a failing coil pack, but that would pop up a check engine light.

Hopefully your mechanic can come up with something.

 

Until the computer finds the timing it injects fuel into all 4 cylinders. If it cannot find the timing the engine fails to start and will not throw a code. Timing is detected by the crankshaft and camshaft sensor so check these with a voltmeter.

 

This just happened to my daughter this evening as she was heading south on I95 at 70mph. Scary to say the least. She managed to pull over and stop. After 2 to 3 mins the car did restart. She continued home, another 75 miles, with no issue. She was shaken. Her car is a 2017 Forester, manual trans, 2.5i base model. The evening was cool at 30F and her tank was 75% full. Once she arrived home, I plugged in my code reader and diagnostic software but there were no codes. I ran a full diagnostic and even checked the alternator and battery with my voltmeter and tester but both were fine. I even put a new battery in for good measure as the one in there had a 9/2017 date. So, I still have not a clue as to what caused this...I did consider the crank or cam sensors but I did not see any codes and I would have expected to see codes as I have in other vehicles -- I recently had a crank sensor issue with an Xterra and the code was plain as day...

I will report to NHTSA as this is actually the second incident for her. The first was not dramatic so she never mentioned it. Oh boy!

 

I had similar issues with the Foz wanting to stall out after exiting a highway at a red light, and one actual stall and no start requiring a tow. I had no malfunction codes and the next day the car started up with no problem. I brought it to the local dealership, and the tech could not duplicate the issue but did suggest that an upper intake cleaning could help. Turned out to be the solution, as mine now no longer has issues after a heated run on the highway. Might be worth considering if it has never been done.
Otherwise, it seems to me I recall someone posting about cleaning the contact for the crankshaft sensor. On a previous Nissan, it was quite possible to have a stall and no start with no code set when the crankshaft sensor was failing. Good luck getting to the bottom of the issue.

 

I had similar issues with the Foz wanting to stall out after exiting a highway at a red light, and one actual stall and no start requiring a tow. I had no malfunction codes and the next day the car started up with no problem. I brought it to the local dealership, and the tech could not duplicate the issue but did suggest that an upper intake cleaning could help. Turned out to be the solution, as mine now no longer has issues after a heated run on the highway. Might be worth considering if it has never been done.
Otherwise, it seems to me I recall someone posting about cleaning the contact for the crankshaft sensor. On a previous Nissan, it was quite possible to have a stall and no start with no code set when the crankshaft sensor was failing. Good luck getting to the bottom of the issue.

Thanks for the comments. Great minds think alike, I guess. I actually did order the crank and cam sensors for the car today. I had a crank sensor failure on my son's Nissan Xterra which lead to a highway stall at speed. The only difference was that the ECU did throw a code and directed me to the problem. I also have read about fuel pump issues leading to complete engine stall on Subaru's as outlined in the article below:

Subaru Recalls More Than 165,000 Cars and SUVs to Fix Stalling Engines - Consumer Reports

Consumer Reports has the latest on a fuel pump recall involving newer Subaru vehicles, including the Ascent, Forester, Legacy, and Outback.

www.consumerreports.org

Earlier this year, the class action has sought extending the class to include 2017 Foresters. Unfortunately, I cannot wait for that outcome so I am looking to replace the pump on my own and then ask Subaru for re-imbursement via a legal letter. Seemed like a simple job until I saw the cost of the Subaru fuel pump assembly. Mother of God, how can anyone justify spending over $600 for an OEM unit and what was Subaru thinking about by engineering such a costly assembly in the first place. In addition, they are now suggesting it be replaced every 70,000 miles. OMG! Engineers gone bad or oblivious to the added cost of the assembly vs the lack of any tangible benefit.

BTW, the 2016 fuel pump assembly seems to be much less expensive, any idea if they are interchangeable? UPDATE: I found the pump aftermarket for about $200, thankfully. I also found the cross reference for the pump for 2014 thru 2018 as follows:

Electric Fuel Pump
Part Number: 42021SG070
Supersession(s): 42021SG010; 42021VA010