Cannabis does effect short-term memory.
If you work closely with a smoker, you will find them forgetting all kinds of stuff. They simply have no recollection of short conversations.
You also notice that when they have periods of abstinence form smoking their memory magically improves.
I have experienced this from both sides.
When I was in my early 20s I smoked a few times a week, and people always commented on how poor my memory was. I often assumed that they were simply lying because I had no recollection of their 30 second meeting by the water cooler a week ago.
When I gave up regular smoking, people seemed to notice my memory improve within a couple of weeks.
From the other side, I have worked with regular smokers and seen exactly the same patterns.
When people say it rots your brain, there is an element of truth there.