You surely know best, or ...

Please don't mix the topics. Reply into the right one, regarding those parts.

No clearing of cart is a different issue, than cart being stored as it should.

It's actually related to the post you split off and the way it's happening. I create a new sale in the front end, and the cart appears to be empty.
I add one item to the cart and it then shows that item and a previous item from a previous sale. I think your right about the top right cart just needing to refresh the browser, but that then shows the 2 items when one is a current sale and one is from a previous sale.

Adding the notification would help inform the customer that there were previous items in the cart.
In my case, I am seeing this issue for sales that were already completed and appearing in the next sale.

This is happening on 2 different stores with two separate installations.
I'll try to play around with it more and switch themes and see if if I can get a different result turning on/off modules.

The saving of items in cart to a customer being logged in is normal, and expected.

The only issue I see here is that the header cart doesn't refresh automatically, you have to force refresh through browser, or go to next step to see it. The total down in left corner seems correct.

Or yes, there is a second issue in this. The customer should get a visible notification, telling the cart was merged with previously saved item(s). That's the norm from other systems.

Edit! Added issue to GH: https://github.com/arastta/arastta/issues/706

Well I think I found a glitch of some sort.
If I complete the address portion of the checkout, the cart info in the top right corner changes to then show 2 items.

You can see in the next screenshot the 2 items.

Seems an item was in the cart from a previous visit, but it doesn't appear on another visit when doing another transaction.
Only appeared after filling in the address info.