I haven’t pinpointed the exact issue that causes this. I guess it has something to do with GTM and some sort of automated checking of tags as they seem to be very consistent. I don’t think it’s debug mode.
Most people notice this because the “referrer” or “source/medium” is gtm-msr.appspot.com but actually the domain/hostname is also gtm-msr.appspot.com.
There’s a setting under “Administration” → “Your Site Name” → “Data Collection” → “Collect data only from known sites”
If you check that you stop collecting data from domains you haven’t explicitly specified. You’re whitelisting what domains you can collect data from.
Make sure that you add all you sites and subdomains to the “Basic Info” up top on “Data Collection” so that you still collect the data from the sites you actually want to collect data about.
I recommend that you double check what sites you’re currently collecting data from by going to “Analytics” → “Custom reports” → “Add a report” → “Flat Table” and creating a report like this.
The hostname parameter is available under the “Page URL” dimension, just click the dropdown and select “Page URL (domain)”. Add all of these websites, exactly as they are in the table to your whitelist at the top of “Data Collection” in your administration module, with or without “www” and such.
I wrote up this guide as it’s happened to several of my sites that aren’t SPAs. If you have an SPA do a bit more research.