Without the select tool enabled, clicking on an event (rectangular area on a track) will select it, and deselect any previously selected event. This behaviour is expected.
With the select tool enabled, I expect that clicking on an unselected event will add it to the current selection. Unfortunately, previously selected events are being cleared in this case also. I think this behaviour change may have been introduced with the 3.5.1 update.
The issue can be worked around by using drag-select repeatedly for each event you want to add to the current selection, but simply clicking on an event would be much easier.
This functionality is particularly useful for copy/pasting multiple events in a single operation.