Currently unable to reproduce this issue. I have sought additional feedback from the reporter to check some details.
If anyone would like to help reproduce the issue, if you enable the NoCaptcha ReCAPTCHA system, do you get a server error when attempting to take an action that requires completing a CAPTCHA? This may require using Tor, and may be specific to certain exit nodes. My theory involves the IP as seen by the server, but I'm awaiting confirmation of that.
A month ot two ago, I got a bunch of 'query validation failed' from the reCaptcha but haven't kept the logs. I'll keep an eye out for them, but I haven't had a single legitimate Tor signup so mostly it is spambot attempting to signup.