Thanks for bringing that to our attention; it probably changed when we recently made the new cart standard for all "fallback" carts (when the cart appears full-page, rather than as an overlay within your own page). I have referred the issue to Development for investigation.
When our classic cart displayed full-page in the same window/tab as your sales page, our classic cart's Continue Shopping button merely worked as a Back button, just as if the buyer clicked the Back button in their browser. This was kinda clumsy, as it could result in simply reloading the cart if the last thing the buyer did was edit their cart (e.g., remove an item, change quantity, enter a shipping location or discount code, etc.)
However, a better solution for you would be to simply change
target="ej_ejc" (or add the latter if there is no current
target attribute) in all of your cart button codes; this will make the cart open in a separate window/tab, and will re-use that same cart window/tab if it's already open.