Hello Nitay
This is an excellent question. Yes, even though network 3.3.3.3 is considered a stub network in the LSDB it is not removed by the prefix suppression feature. However, the 192.168.34.0 network which is also indicated as a stub network in the LSDB is removed.
The difference between the two is that 192.168.34.0 is also indicated as a point to point network with a neighboring router ID of 4.4.4.4 and a next hop ID of 192.168.34.3. So the router knows that although the network itself is configured as a stub network when viewed as a destination, it is also part of a point to point network. The loopback network 3.3.3.3 which is a stub network does not have a corresponding point to point component, thus, it is not removed.
It is only stub networks that appear in the LSDB that are part of a point to point network that are removed since they are never used as a destination by user traffic.
I hope this has been helpful!
Laz