If you have Invert Colors enabled system-wide and VoiceOver enabled, you are likely to find that the App Switcher cannot be activated.
If you use per-app settings to enable Invert Colors for specific apps, you will likely find that the App Switcher will stop working when you open one of these apps.
When this issue presents itself, your device will still provide the normal and expected feedback whilst performing the gesture on iPhones without a Home button to activate the App Switcher, but it will not be activated. Additionally, if you have assigned the Back Tap or a touch gesture to activate the App Switcher, these also won't work if you are effected by this issue. If triggered by an app set to use Invert Colors, the behavior will persist until you restart your iPhone. Turning VoiceOver off and then on again does not return the App Switcher to normal behavior.
We do not believe the issue to be present when VoiceOver is not enabled along with Invert Colors.