I'm using a wireless adapter from 8BitDo to connect a Wii U Pro controller to my Windows 10 computer.

So far so good.

In the  Win 10 controller configuration/test (joy.cpl) all buttons and sticks work without a problem.

Same on gamepad-tester.com

Everything works.

But when I try to use it on a steam game, the directions left/right on both sticks don't register at all. Up/down works, but not left/right.

Has anybody ever had a similar problem and maybe a fix?

It's a perfectly working and good controller, but this is driving me mad, because I don't understand why left/right won't work.

(I already tried re-installing the device/driver, no luck.)

Thanks in advance!

You may have to go into the controller settings within Steam and select that you're using a Switch controller or generic controller.

Go to Steam (upper left hand corner)>Settings>Controller>General Controller Settings 

Steam doesn't always do the best job of automatically detecting what controller you're using.

Also, since you're using a Wii U pro controller, make sure the game you're playing has "Full Controller Support." If the games Steam page has it listed as Partial Controller Support or doesn't say at all, that also might be the reason it's not working correctly.

Thanks for the quick reply.

Steam recognizes it as a generic controller, and the game has full controller support.

Inside the Steam calibration, when I try to rotate the sticks, only up and down register. It's like left/right doesn't exist, and I can't for the life of me figure out why.

Definitely a Steam setting issue!

Here's a tip that I found that you can give a try

Launch Big Picture Mode from Steam>Library>select the game you're trying to play>Manage Game>Controller Options>From the dropdown menu, select Forced Off and save everything.

It'll restart Steam afterwards and give it a try. This was a tip I found in the Steam forums where someone was complaining about their joystick not working properly with Steam games. It worked flawlessly with the non-Steam version of a game, but not the Steam version. So this sounds very similar to your issue.

Does it work properly when you plug it in via cable?

If it still doesn't work after that, I would have Steam recognize your Wii U pro controller as a Switch controller specifically instead of a generic controller.

Other than that, I'm not really sure what other troubleshooting steps can be taken.

I clicked around a lot and now somehow it showed up as an XBox controller for a while.
I noticed that in the configuration somehow two axes showed up as "---".
No idea what I did, but now the left stick does register left/right.
Now for the stubborn right stick...

While I'm here, are there other controllers that work well with Windows 10/11?
I think that an Xbox Series X controller should work out of the box... Does anybody use one of those on PC and can tell me if it's hassle-free?

Yeah, what Yuri suggested of making your controller wired to see if it works that way should've been troubleshooting step number 1

I have a Series X controller with the Xbox Wireless Adapter for Windows and it works flawlessly.
https://www.amazon.com/Microsoft-Xbox-Wireless-Adapter-Windows-one/dp/B00ZB7W4QU

Some games are just shit at configurations. Binary Domain is broken as shit for controller support. Xbox or PlayStation controllers I just could not get them to work with a proper configuration. I jumped through a bunch of hoops and the only controller I could get working right was the Switch Pro Controller on a 2012 PC game. Go figure. Some games only offer partial controller support.

It seems that way. Plugging it in did nothing.

But after a lot of fiddling in the settings I finally got it to work. For both the sticks the left/right axis wasn't defined for some reason.

I hope that it properly saved those settings now.

Thanks everyone for the suggestions.