It's actually ok, they do load. I forgot that the infinite scroll system still exists, and I had not actually tested the real case with > 50 items.

It turns out my bug only happens in development:
If you edit the Planning v2 code to fix the maximum number of items (limit) to fetch at once to 2, and given you have a tall enough viewport, then the infinite scroll will never kick in, because the 2 elements do not take enough screen height, therefore there is no scroll bar, therefore the infinite scroll never starts.

It does not happen in production because the limit is set to 50 items, which take some space, and people do not usually browse Tuleap on giant screen resolutions, so 50 items add a scroll bar, and so the infinite scroller works.