List view strange behavior SharePoint Online

Hello everyone,

We have two test lists that has more than 5000 items. Below are the cases -

List A - All item view does not work and we see List Throttling error.

List B - All items view works fine with scrolling option. We see list throttling error when "Count" is being used on one of the fields.

Not sure why this scrolling option works in List B and why not List A. Any insights is very much helpful.