Problem with selecting/deselecting behavior


I used to do 3D with other softwares and now I’m learning how to use Blender.

This is with 2.80.

I don’t know what I’m doing wrong but there is something very puzzling with the interface for me when in object mode.
Normally if I have something selected (whether a vertex, an edge or a face) if I select something else then this first one will be deselected.
And if I want the first one to stay selected while I select the second one then I have to hold shift which is what I expect.

So far so good.

But now if I do the same thing while I have the transform option selected on the left, and the gizmo appears then it works the other way :
it won’t deselect the first when I select something else = it’s just like I had been holding shift (and I haven’t)… I have to do AA then B so it deselects all and then is able to select again…
It’s super puzzling.

I’ve searched the preferences, the keymaps, lots of places : I can’t seem to find a way so the behavior is the same whether I have the transform tool/gizmo selected or not.
Maybe by mistake I did a bad ctrl-key and didn’t notice it toggled something in the background… I don’t know…

Anyone has an idea please ?

Thanks in advance !

-Frédéric (France)