I have heard many stories of their batteries popping out of the spark and it seems like an increasing concern. What is the best soluiton for this? And has DJI taken an action on fixing it?

https://forum.dji.com/forum.php?mod=viewthread&tid=198914

I think it happens when people don't notice that it's not clicked in fully. I always check for that and have not had one fall out. Yet.

Hi, thank you for bringing this query for us. We can guarantee that the DJI Spark will fly normally if the battery is inserted correctly and it is in good condition. You may also try using a battery clip on your DJI Spark as long as it will not affect the flight performance and stability of the said drone. Please don't hesitate to reach us if you have any inquiries or concerns. Thank you for your continued support!

After you have installed the battery give it a tug to make sure it is secure.  I also (usually) us an o-ring battery clip for extra safety.

Don't buy a Spark  

No action taken by DJI, just get a battery clip. I wouldn't fly my spark without one!

MccloudSpark Posted at 10-3 19:42
After you have installed the battery give it a tug to make sure it is secure.  I also (usually) us an o-ring battery clip for extra safety.

Since my encounter of part time "pregnancy", I am almost sure it is not so much a user fault, like it is the battery releasing itself through the belly swelling, midair.
Heavy usage of the batteries, the best way would be to shut a Lipo  off before or at least at around 20%, but I doubt that all pilots do that all the time, lets age 'em quicker.
I had, some times, trouble to get the "warm" one out easy, for a quick change after a (batt) stressy flight, also hat once or twice  trouble to get a warm one in, direct after charging, thats why I decidet to get me the clips.
I flew with rubber for some time, and  in case I wanted to use the guards, I would have to do that again, but either way, as long the rubber is tight, that should be a help for the pilots