Over a few frames things settle down and the driver is no longer allocating new blocks of memory but is instead just handing back blocks of memory that had previously been used. So in other words it's not necessary to do your own multi-buffering, because the driver itself is automatically multi-buffering for you behind-the-scenes.

At this stage it's worth highlighting that this buffer update model is well-known and widely-used in D3D-land (where it's called "discard/no-overwrite") and has existed since D3D8, if not earlier; i.e it has close on 15 years of real-world usage behind it. So it's not some kind of voodoo magic that you may not be able to rely on; it's a well-known and widely-understood usage pattern that driver writers anticipate and optimize around.

Both DX12 and GL4 are moving away from this pattern and moving towards an explicit low level access memory management. With fences, unsynchronized access, and persistent mapping.

Drivers may optimize for the discard/map-no-overwrite pattern, but the higher level app. has much more information than the driver on how to deal with memory and access hazards. Driver optimizations can only go so far.
But with great power, comes great responsability.