[JRSCCivic98]

That's not it. Your message options are set to expire and delete messages that are more then 30 days old. So every day at midnight, anything that's 31 days old gets deleted and hence decreases your message count. You can set it to never expire. I know you set it to never, but this is the only setting that would do this. Double check it and also make sure you're on the latest os.