So, safari in ipadOS (that's the only place I use safari) works the same way, and it's as annoying as it sounds.

I would say it should be the opposite. Separate windows should be independent but tabs on other hand can share same session.

yeah that seems intended and obvious to me as well

In fact, that's a good way to understand the behavior. Log into a site. Visit the site in your other window. If they share a session, you'll be logged in. If they don't, you'll be logged out.

Wish it was

There are legitimate reasons. You may have a main account in a given page which you usually want to log into automatically, but at certain occasions you might want to use an alt without your browser forgetting to log into your main by default. A more specific example of this would be someone who has an alt account for NSFW stuff in social media because they want to be able to browse in their main without risk of genitals popping up on their screen while they're on the bus or using the phone next to family.

I use it when I need to log into a computer at work with a personal account, so that I know I'm logged out when I close the window.

I use it to test that I've set up an authentication system correctly without a cookie bias, among other uses

Set everything up in main > confirm tests pass > log in from incognito with password vault to make sure the auto test didn't lie.

anonymity is not the only usage for incognito, and frankly it doesn’t even make you very anonymous on its own

Signing in to a service would defeat incognito mode if its function were to make you incognito towards the websites you visit.

But since that's not what it does, I don't see why it would defeat anything.

Logging into alt accounts without logging out of main

Yes

For Firefox users who don't know - try right clicking on the new tab button...

go on pornhub

dont close the window, just hide it afterwards

join a meeting, share the screen and type in „po” because you need to go to a website starting with po

pornhub shows up

get fired because youre jerking of with company property

One example of being more careful about privacy that came up for me recently is that I have gmail as my primary mail and I use google docs for storing a lot of things I use for work. If I also want to block youtube ads and they say it's against their TOS, they can theoretically close my gmail and docs for violating it.

They might not and I haven't necessarily seen anything saying they're moving in that direction, but since last month I've been exclusively watching yt either through proxy sites and apps or opening it in incog windows where I'm logged out.