Issue with HC-SR505 sensor
You asked if anyone else has experienced this behavior, and from the documentation I can find the answer must be yes.  

The 505 holds the HIGH output for a certain amount of time.  None of the documentation I can find on it specifies the amount of time.  So it is very possible that you are operating right near that limit.  If their quality control is not perfect, some might work correctly and some might not.  

In other words, I believe the problem is that you are not allowing for the built-in time delay in the SR505.  

That is the main difference between the 501 and the 505.  On the 501 it is adjustable in both sensitivity and time delay.  With the 505 those values are preset.

As a workaround, you can try increasing your delay upwards from 2000ms until you find the right value that will work consistently with your batch of them.

The sensitivity cannot be adjusted, either, but this can be an advantage if you are trying to cover a smaller area than you would try with the 501.  They both have the same 90-100 degree width of view, but the 505 is made for shorter range like a doorway, as opposed to the 501 which can cover an entire room.