In case it is confusing; I pause and unpause the game in the video, which is why the flashing of those pink icons changes through the video.
I resized some UI elements and thought I solved the problem, but it was just the single tile icons were sized properly and did not flash. The problem happens with any icons that are multiple tiles in size (the tiles of the inventory grid).
When anything is added, removed, or moved in the inventory, it is refreshed; new item widgets are created and their icons are set to their desired size. It seems to be this part that is causing the flashing. A widget begins as the single tile size and quickly resized to match the items desired slot size. For some reason, when the game is paused (using Set Game Paused) this flashing happens, and it does not happen when the game is not paused. Nothing is happening on tick in the inventory menu, so tick when paused should not be the issue. I don’t know what to try.