I'm using the mini regulator with 45 gram co2 cartridges that I just purchased from Williams.
With the water nicely carbonated after 2 co2 cartridges emptied into the keg (overnight) I added another cartridge and set it at 5 PSI and left it. It also emptied out overnight making me think that there is something wrong with the regulator itself.
I e-mailed Williams about this yesterday morning and have not heard back yet.
I also e-mailed them about leaving the regulator set at serving pressure several days ago and never got a reply to that e-mail which is surprising as I've always been very happy with their prompt customer service.
Maybe it went to spam? Give them a call: https://www.williamsbrewing.com/Customer-Service-W7C197.aspx
I've been a little wary of the mini-regulators, even though they would be a good fit for a small-batch brewer like me. I hope you get this problem solved because they are a good fit for small-batch brewers.