Comment by insin

If you're using an adblocker, it's because YouTube video grid items have an [is-in-first-column] attribute which gives them extra margin-left, throwing off alignment when videos flow to fill in gaps created by promoted videos which were hidden.

It's kind of silly that they add these attributes to each nth item based on what they expect the grid width to be, when you can get the same layout without them (my YouTube extension mentioned elsewhere in this thread performs this style fix so grid items line up properly when videos and entire cross-cutting shelves are hidden and the rest flow to fill in the gaps), but I suppose they have no incentive to make the layout work when videos are being hidden or the grid is otherwise being modified externally to work in a way they didn't want.