Hi there! I never turned Captcha back on, and no one has had issues since, so it isn't still happening. (I've had several signups since turning it off, and they were by people who know me so I was able to check after the fact.)
Update: with a little testing I've been able to confirm Akismet is NOT the problem. I'm guessing mobile browsers (at least Chrome and Edge on Android) are getting hung up on the javascript part of the Captcha, maybe? I don't know, but I can say things are working for my newsletter once I turn off Captcha in the anti-spam settings under the subscription settings.