Being asked for a passcode BEFORE downloading a iOS update on iPad Pro

This is likely to be because you have enabled Automatic Updates in your Settings. If you disable Automatic Updates, therefore reverting to manual updates, you are unlikely to see this specific challenge.

iOS/iPadOS has always presented a challenge for your device Passcode at the point of manually installing an OS update. As such, this unexpected challenge may simply be an artefact of selecting Automatic in preference to Manual.

I hope this clarification is helpful to you.

That process has been the case for all my recent updates.

I’ve never known that not to be the case. Whenever manually updating over WiFi, I’ve always had to input my screen lock passcode for the device.