Unwanted auto-replies in iMessage "Auto Generated Text:OK"

For the past six months or so, iMessage on my iPhone X (running iOS 13.5.1) has occasionally auto-replied to incoming text messages with "Auto Generated Text:OK" Does anyone know how to prevent that from happening? The unwanted auto reply recently responded to an incoming pharmacy text message with "Auto Generated Text:OK" causing the pharmacy to think I wanted to refill a prescription. Ouch. Apple Support thinks these unwanted auto-replies may be coming from a third party-app. I've searched my phone for rogue apps that may be accessing iMessage...but so far no luck. (Note that my Do Not Disturb is set for Auto Reply to No One)