Hi @gumm
ACF will only load 20 users when you search the users. It’s possible that your server can’t handle the query for a lot of users. Could you please test it out by cloning your site to another server or your local machine that have higher specifications?
If the issue persists, could you please open a new ticket here: https://support.advancedcustomfields.com/new-ticket? Please provide temporary admin credentials and explain the issue again in that ticket.
Thanks 🙂