
Based on your description, I understand that when you use Group By function in the list, you cannot see all items in the group, you need to click "Show All" to see all items in the group.
I tested this and also meet the same symptom as your description. Any group that has more than 30 items in it now only shows the first 30 items, with a 'Show All' link at the bottom.
It's by design in the modern view. And there are no settings to change this behavior.
As a workaround, if possible, you could return to classic view to see all Group By items.
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