Is that the same one that sometimes asks you to click on motor cycles and zebra stripes in a loop and you never come out? (this happens often especially if you're on a somewhat uncommon IP address, like an IPv6 /48 with just a few customers on it)
That’s V2. V3 is always invisible and only returns a score. It’s not unusual to redirect low scorers to a v2 challenge, but afaict you have to set that up by hand.
Make sure to have good KPIs not using the pure number of accounts. I think its absolutly valid to have it like that.
IF you really don't want to use a captcha, why not write something very basic specific to your page?
Those are probably spam bots filling out a basic form.