I’ve had this same issue w an ipad pro 11 (2018). Seems related to dictation. When dictation is turned off, it stops for good. Once re-enabled and used, it starts again and doesn't stop unless reset or restart.
You have to activate Siri (for example with the home button) to stop it. Often a couple of times in a row. I get the impression that you have to activate Siri as often as you used voice dictation before. Sometimes it tooks me 15 or so Siri activations in a row to get rid of the spinning wheel (right before that I used voice dictation in a WhatsApp chat many times).
Well, I’ve just stopped using Siri dictation on my iPad so I don’t have to worry about it. none of these work-around are desirable, I wish there was a permanent fix.