There is also a word sandbox in the html source code when this checkout url changes. I guess this is some kind of experiment they are running. And Shopify support team has no clue.
I reached out to Shopify support, and after initially misunderstanding me (they thought I wanted to change the order of my payment gateways which I know is not possible), they came back with the following explanation:
I checked this further with our Technical Team and they were able to inform me that this is a new checkout process that is being rolled out across Shopify.
This explains why you are only seeing this checkout every so often and is not happening every time. This does not affect the actual transaction and we are working at making this transition as smooth as possible.
The reason behind this new checkout process is to help reduce confusion, buyers will now see a temporary Confirmation Number on the Thank You page after an order is placed. The confirmation number is a randomized, non-sequential alphanumeric serial.