session – Cart lost for logged in customer. Retrieved only after adding item

We run a B2B webshop in Magento 2.3.5 and customers have to log in to be able to add items to the cart (Amasty Hide Price-module). However we are experiencing problems with disappearing carts:

  1. Customer logs in
  2. Customer places several items in cart.
  3. Customer logs out / is logged out after some time.
  4. Customer logs back in: cart is empty.
  5. Customer places random item in cart: this item is in the cart together with the previously entered items.

I think it is a session / cookie issue but I can’t quite figure out what / why this is happening and how to solve it. Any help would be greatly appreciated!