90% traffic drop when cookie banner implemented on Joomla

I wonder if anyone with experience of Joomla can share any insight? One of my clients migrated to the platform earlier in the year. When the cookie banner was implemented, and tracking set up via Tag Manager, traffic dropped by around 90% YOY. I know a 30-60% drop is normal (and is what I see with other clients on other platforms), but 90% seems an insane drop. Stats on Search Console are showing a fairly steady/gradual increase in traffic over the year, so I don't think there has actually been any significant change in traffic to the website. I've checked the Tag Manager setup and all seems fine. I've even reimplemented it to make sure. The cookie banner is in the centre of the page, and I've tried different formatting to try and drive people to consent. The acceptance logs on Joomla also show a vast majority of IP addresses coming from non-UK sources (mostly US) - very unlikely based on the industry my client is in and the area they serve. And the same logs show every cookie as denied over the past month. Which can't be true as I myself have accepted cookies in that time, and analytics is still showing traffic. I've worked with this client for a number of years and had really excellent results but this is my first time working with Joomla. It's very frustrating that the data I'm sharing with them just looks like they're seeing a constant loss. They're on Joomla 4 (due to upgrade in the new year). The cookie banner plugin is just called Cookies Policy Notification Bar. Has anyone else had a comparative experience in Joomla? Or am I unfairly laying the blame at the platform's door?