I use the QuoteFactory in order to load the quote by id & add items. Then I save the quote & call $quote->setTotalsCollectedFlag(false)->collectTotals().
After that, I can see on the database that the quote item have been added & on the frontend I also see the right qty on the cart page, but the mini-cart on the header stays the same.
When I sign out & in again it gets updated.
*My guess is that I need to somehow force the customerData to reload on frontend (customerData.reload(['cart'])), but I don't know how/if it's possible to performs that on the server side...
-- Any idea?