Yes, this is normal behavior for some iPad models. The volume buttons on these iPads are dynamic, meaning that they adjust their function based on how you are holding your device. When you are holding your iPad in landscape mode, the top/left button increases the volume and the bottom/right button decreases the volume. When you are holding your iPad in portrait mode, the top/left button increases the volume and the bottom/right button mutes the sound.
This behavior is different from other iPad models and iPhones, which have static volume buttons that always increase the volume when you press the top button and decrease the volume when you press the bottom button.
The dynamic volume buttons were introduced on the iPad Pro 11-inch (4th generation) and iPad Pro 12.9-inch (6th generation) in 2021. They have since been implemented on other iPad models, including the iPad (10th generation), iPad mini (6th generation), and iPad Air (5th generation).
If you are not used to the dynamic volume buttons, you can disable them in Settings > Accessibility > Touch > Invert Volume Buttons. This will make the top/left button decrease the volume and the bottom/right button increase the volume, regardless of how you are holding your iPad.
I hope this helps!