It seems not to be a rare phenomenon…
After I use Windows Update Assistant upgrade to 20H2 from 2004, I also see many Office tiles appear on the start screen, even though I removed them in the past. Then, I use the original iso from media creation tool to in-place upgrade on another computer, the situation is similar with yours: start menu layout changed and Office icons appear.
I think it should not be a issue, because start menu’s change was just one of the biggest changes in this version(another is new Chromium-based Edge).
Windows 10's Start Menu has changed a lot since the initial release, and 20H2's change is perhaps the biggest yet, as it's intended to show off Microsoft's new Fluent Design icons as well as improving support for Windows 10's Dark and Light modes. Most of Microsoft's own applications(specially Office products) now have icons that support the new modes, and the resulting combination of translucent tiles and new icon designs works surprisingly well.
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