Well, I can confirm it was definitely happening to me in 3.0.2.8.
I have not yet had an opportunity to test it in 3.0.3.x.

Possibly. My Jolla 1 has a teensy-weensy bit less resources

Ah, sad!
Thanks for testing. Can burn my hope now

A curious observation. I was watching a YouTube video in Browser for about 10 minutes and when finished, I navigated to another site (TMO, for those curious ones ). Then I decided to close Browser, which I always do by closing all open tabs to avoid Browser opening them again the next time. I only had one tab open anyway. But, what would you know, tapping on the square in the Browser bottom bar did not bring the list of tabs as expected. Oh no. It brought the unlock screen where you need to tap your PIN.

If I understand it correctly, my screen timed out while I was watching the video but the video playback prevented it. But some flag was set somewhere that the screen had timed out and so the unlock screen was shown at the nearest opportunity.

I have seen it happening on other occasions with other apps that keep the screen alive, such as Pure Maps during navigation. In such cases, navigating to the home screen is greeted with the login prompt. Annoying, as it does not match the user's expectation (the screen has never gone dark, why on earth do you want me to unlock it?), but at least understandable. But this? I never left Browser. Navigating from YouTube to TMO did not prompt me to unlock the screen. Only opening a different view in Browser (the tabs list) did. As if that counted as trying to switch to another application. Positively weird.

Nothing to do with the subject of your post, but I couldn't resist giving a tip to fellow tab closer. There's a setting that the browser closes all tabs on exit automatically: Settings -> Apps -> Browser -> Close all tabs on exit.

I know. I prefer to close my tabs manually, to keep the flexibility of sometimes remembering them for the next time

Has always been the case I think, or ever since I started using Sailfish, from ver 2.x. I thought it to be weird at first. But in some way, it felt logical or quite fine. If you do not interact with screen, it locks the device, but since the video is playing and is in focus, screen does not blank. Access is limited to that particular browser tab. You might have left the video playing and left the phone unattended, which would be foolish I agree, in which case anyone could grab the device and access some already open private tabs or other data unless it is protected as it is now.

Oh crap!! That's torn it LOL
Had to update my Xperia X via terminal, but forgot to un-apply patches
Now can't enable/disable Extra Folder icons and Lockscreen analogue clock. Daaah.
Oh well. Hoping to get an XA2 sometime soon anyway.

Can't you just reinstall the "offending" patches?

No. They show up as installed in the web catalog and won't re-install.
All the other patches still work fine, it's just those two.

That's my bugbear too, Patchmanager refusing to reinstall existing patches. It can upgrade but not reinstall the same version. You have to uninstall them first. If PM refused to dfo that, you can try manually from the CLI.