When I turn my iPad landscape the volume button changes sides

Dynamic volume controls were introduced with release of iPadOS 15.4 onwards. The “logic” behind dynamic volume controls is such that regardless of the screen orientation, the relative function remains constant - as follows:

• When the physical buttons are positioned on a vertical edge of the iPad, the uppermost button will always increase the volume. Conversely, lower button will decrease the volume.

• When the physical buttons are positioned on a horizontal edge of then iPad, the button on the right will increase the volume - while the button on the left will decrease the volume.

For reasons known only to Apple, the ability to disable this feature is not available for any new models of iPad and iPad Pro that have since been launched - these models being:

• iPad Air5
• iPad mini6
• fourth generation iPad Pro 11”
• sixth generation iPad Pro 12.9”
• iPad10

This is documented within this recently updated Apple Support document:

Use the Home, side, and other buttons on your iPad - Apple Support

Apple invite submission of constructive comments and feature requests via its Product Feedback pages. If you would like to see the ability to disable dynamic volume control functions for your iPad, this would be the most appropriate channel through which to make your voice heard:

Feedback - iPad - Apple

The volume buttons are oriented how you would expect them to be, if you don’t assume that the buttons’ functions are always fixed.

This whether the volume buttons are oriented left ⊖ and right ⊕, or up ⊕ and down ⊖.

If you’ve been around Apple UI design changes for a while, this volume button change is analogous to the scrolling direction change that happened on macOS some years ago. Prior to that change (and which is now spreading across all platforms), the scrolling direction was reversed from what you’d expect, looking at the display.

Yes, that's the way it's designed to work.