Who knows why this happens, often and can happen with any app:
In this example which happened today the app Pocket is shown to have an update (right pic), if I tap on the app entry just above "update" to... well, get and install that update, I am presented with a page with "uninstall" and "open" (left pic) as options... not one "update" as it should be. It takes several tries or to wait some time to get the app page with the update buttons.
Who has noticed this behavior? Play store bug? or feature... since the updated file may not just yet be available? happens to me quite often. my workaround is to tap Open then close the app, and the Update button appears after that. pressing the back button then update again gets the job done for me.
I've noticed it and I generally don't have auto update or notify on anymore (on