Hi Domoviye,
Welcome to Microsoft Community Forum. Thank you for sharing your concern with us today and we hope that all is well.
We understand that you are trying to use Xbox Elite Controller - Series 2 in playing games but experience an issue with "A" button; when pressed launches Xbox Game Bar. Let us check on this together so you will get back to gaming.
I have catered an issue similar with what you have described and just a heads up, you may install the new update for the Xbox Accessories app and update your Controller's firmware and that should fix the issue.
How to Update Xbox Accessories app:
Launch Microsoft Store and select ... on top right. Select Downloads and updates. Look for Xbox Accessories app and download update.
How to update your controller on a Windows 10 PC by using the Xbox Accessories app:
If you don’t already have the app installed, get it in the: Microsoft Store
Open the Xbox Accessories app.
Connect your Xbox Wireless Controller by using a USB cable or the Xbox Wireless Adapter for Windows.
Note You'll be prompted to update to Windows 10 Anniversary Edition if you haven't yet done so.
When the controller is connected, you'll see the message Update Required if an update is mandatory. Install the update.
The Xbox Accessories app is available only on Windows 10.
Controllers that don't have a built-in 3.5-mm audio port must be updated using a USB connection and must have the Windows 10 TH2 Update (or later).
Updating the controller over Bluetooth is not supported.
To customize the Xbox Elite Wireless Controller Series 2 in the Xbox Accessories app, get the Windows 10 May 2019 Update (or later) and make sure your PC has the latest fixes. See how to get updates
Bluetooth requires a PC running the Windows 10 Anniversary Update. To learn how to get the latest Windows update, see: Windows Update: FAQ
Resources: Elite Controller A button brings up Xbox game bar on PC : xboxone
If this is not resolved, you may try to restart PC and power cycle your controller:
Press and hold the Xbox button for 5-10 seconds, until the controller turns off.
Wait a few seconds, then press and hold the Xbox button again to turn the controller back on.
Previous temporary resolution:
Kindly launch Settings on PC. Go to Settings > Gaming > uncheck box for "Open Xbox Game Bar using this button on a controller."
Or connect Controller to PC with USB cable.
Please, do not forget to check here for known issues that might be related to your concern.
Xbox Status
Known Issues for Game Launches
Kindly let us know, if there is any misunderstanding of our description by sending us a reply.
We look forward to your response. If you have other clarifications about this matter, you can get back to us by replying to this post.
Sincerely,
Kathy A.
Xbox Forum Moderator
Microsoft Community