The App Switcher thumbnail images are cached - the individual thumbnail image usually being updated as you switch away from the current foreground App.
There appears to be a bug in iPadOS that, when available system resources are being fully utilised, the thumbnail images might not be updated as expected. As the thumbnail image is in most cases too small to show detail, this largely cosmetic detail is of low consequence.
Force-closing unused Apps is usually unnecessary, however doing so may release sufficient resources for the thumbnail image to refresh when the associated App is next accessed.