I tried to select many objects on different zoom levels with alt+select. and this is what i found out:
The delete button is not working some of the time (so not always) and mostly the times I'm more zoomed in, Although it also sometimes happens when I'm zoomed out a lot, but rarely. So I do not think it is because of the number of selected objects alone. I think it is more of the combination of selected objects, zoom level, and way of selecting (alt+select). But I can't find the relationship between them. It so uncertain.
Hopefully we can solve this and find why it is happening.