vspot415 much like a system reset on the phone wont't solve the issue (likely) I'm not confident that replacing the Infotainment will either. I suppose it can if there is a bug is some production lines but that seems unlikely. In the 5 months I've had the my CX5 I've had both a Samsung Galaxy S7 Edge AND my newer Samsung Note 8 BOTH have the intermittent problem. And I don't use the native messaging app, I use TEXTRA. I was going to test using a different messaging app but seems as though that's unnecessary since you already use something different. FWIW my S7 is direct from Sprint (unlocked). My Note 8 was purchased on Swappa but the previous owner was on Sprint as well.
Mazda saying they've never heard of this problem is Mazda not being very diligent on logging details of their calls as I've called them about this (in fairness with a few other issues) and have a Case number. I've tried 3 different OS versions, the most recent one with the installation of Android Auto. The only thing they mentioned is that the phone(s) were not on their official list of supported phones (!). How a 2 plus year old Samsung Galaxy phone is still not supported is another issue but got the line "we are adding support all the time" line.
Based on our similarities and differences it appears as though it's not specific to carrier (Sprint) or application (stock, Textra) or firmware version of infotainment I've updated the patch level but both of my phones are running Android 8.0 (Oreo) so I can't speak to a version prior (Nougat). What version are you using? If it's Oreo maybe if you did reset to a prior version it would be worth it to see. Saving that my only thought is it's something with Samsung Galaxy S7 and above and just right now there aren't enough people that have spoken out on the issue? Is there anyone on this forum who is running S7/S8/S9 that is not experiencing this?