Multisite ‘accept’ doesn’t go away when /fr, but works /fr/

I have a WP multisite with a site for each language. The main site works fine domain.com, but when I go to a other site domain.com/fr the popup doesn’t go away, I see a cookie being created (3-wpgdprc-consent-23, accept), but the popup keeps being displayed.

When I go to the same site with with a trailing slash, domain.com/fr/ everything works as expected. So my solution for now is redirecting /fr to /fr/ and this works for now, but this seems like a bug.

I run WP rocket on the site, but I’ve tried disabling this, but that doesn’t have any effect. There are three sites in total and they all run the same theme and plugins. The main domain works fine (domain.com) and all sub sites (/nl and /fr) have this issue.

As I said I have redirected both domains to include a trailing slash so the issue is fixed for now, but I would like to know what is causing this issue.