I have noticed something similar with my car’s infotainment system (not Android Auto). I most regularly run into it when I’m taking voice notes in Google Keep. AntennaPod dutifully pauses when I start making a voice note, but often doesn’t resume when I stop recording. I hadn’t noticed a correlation with a particular time threshold (such as the 30 seconds noted here).
I just did a test talking directly to the phone with no Bluetooth or other devices involved. It worked exactly as explained: AntennaPod did not resume at the end of my first voice note of somewhere around 30 seconds, but did resume after a deliberately shorter voice note of less than 30 seconds.