Have you noticed an increase in their sign ups the past week?
I have xenForo 2.1 installed on all my boards, and we have the same issue, even with reCaptcha 2. I really wish xenForo installed reCaptcha 3 to see if it solves any of the issues. I'll turn on SolveMedia again, soon. I wanted them to get the issues ironed out before proceeding. It was having server errors sometimes.
Google's server reliability is top-notch, but they are not very good on other areas. reCaptcha is one of them.
What I did is put honeypot/stopforumspam API's onto my board so it would mitigate the spam for me. I see the approval queue with 3 to 10 per week, but if they come through, I just IP discourage/ban their accounts. Keep doing it until it goes to a respectable level of comfort. The more you block their access, the more they start thinking "This site has an active admin." But worse than that, if they spent money to buy accounts, they e-mail you asking you to pay them back. I'm looking at those e-mails with "What the fuck? You paid to post on my board? HAHAHAHA.hahahaha. What an idiot."
No, really,
that did happen. People register, sell them, try to link on the site in question, and loses because I'm the active admin. Notice, that every time they do, there's a new IP address after they register? It's a spammer.