Hi, welcome to the world of Project. The values in the resource availability are used in the intial assignment only, and are not retrospectively applied – plus they dedine the units used before the resource is defined as overallocated.
A better way model this is to edit the resouce using the Resource Information dialogue box, and set the the resource calendar to be non-working for when the resource is not available. The task will now move (assuming you have not constrained it).
As can be seen from the screen shot below, I’ve mocked this up where Resource R1 has a weeks non-working time defined in their calendar. The Task T1 gets extended by a week when this non-working time is entered, but you can see the work of 80 remains uneffected.