In addition to cached redirects, HTTP Strict Transport Security (aka HSTS) may be at play. HSTS is a security feature that forces the browser to use HTTPS even when accessing an HTTP URL.

The browser will start using HSTS for a domain after receiving a Strict-Transport-Security header from the server. The browser also ships with a list of domains for which HSTS is enabled by default.

In Chrome, there is a way to delete your domain from HSTS after it was added by the server. Though, you can’t exclude domains that are baked in the browser (this includes popular websites and notably everything under the new .dev TLD)

Go to chrome://net-internals/#hsts. Enter example.com under Delete domain security policies and press the Delete button.

Now go to chrome://settings/clearBrowserData, tick the box Cached images and files and press click the button Clear data.

already known bug. And very nasty. As i can see internally no fix ready yet.

Unofficial workaround:
Open in Vivaldi's address field chrome://flags/#https-upgrades
Set to Disable
Restart Vivaldi

⚠ Flags settings are always very experimental, unofficial and can cause other issues.

I hope you can remember much later that you made such changes when something is wrong.

for someone who uses very dark colours for my theme and has a lot of tabs opened at once, I have a hard time distinguishing between which tab I'm currently on.

It would be nice to have a implementation of a setting that adds a subtle but visible border around the currently active tab. This would provide a stronger visual cue, making it easier to identify the active tab at a glance, especially in situations with many tabs open and dark coloured themes.