Poser 6 does not introduce the problem.
Poser 7 does.

If you take a PZ3 saved by Poser 6, then open it in Poser 7, then save it, that PZ3 will have the importing issues.

Now I removed my P6 when I installed P7, but I suppose that if you have PZ3 saved by P7, then open it in P6, then save it, that those PZ3 will work fine, as intended.

"If compression has nothing to do with then you explain to me why the prop comes in after I do that, and I'm really not having a go, I can't explain it!!"
Because compressed files do not import into Vue.  You have already stated that some of your files are compressed.  That is a different and independent problem.

You need to isolate which problem you are having.

Let me give you an analogy.
Let us say your car is overheating.
Now lets for example say that the problem could be a hole in the radiator, or the fan belt is broken.  Now the problem with the overheating could also be BOTH occuring at the same time, but that does not mean they are related problems(or the same cause, that is).

Either of them(or both simultaneously) are going to cause the same condition, the car overheating.  You will have to isolate the problem(especially if you are going to offer advice to others, when their car is overheating too).

A compressed file AND an uncompressed file written with the wrong path to the object are BOTH going to look the same, that is no files will be imported.  They look the same, but they are unrelated.

Now, Looking at the Python script for the uncompression routine, there are calls to rewrite the directory paths in the PZ3.  I am not a Python programmer, but it may be possible that this routine is introducing new code into your mysteriously compressed files to point them to the object.  I don't know, I am unable to simulate the (compressed/zip/Z)conditions you are describing.  I am unable to isolate THAT particular variable. 

Other than the fact that when I ran the script on an offending file, it did not fix the problem with importing the object file.  That was not a 'Nyaa Nyaa Nyaa', that was a step in isolating the problem.

That is why, I gave you an offending control prop file, that you can get to, to test.  To help isolate the problems.  Is it the fan belt, or the radiator?  I don't have access to your files to test your situation(you can describe them and the pictures you have made with them, I don't have access to those files).  I  have given you access to test files which I have issue with, but you don't want to test. /shrug. 

If you don't want to pop the hood, to see if it is the fan belt or the radiator, I can't force you to.

The file is right there(it's free, and you don't even have to log in or register to get it), run your solution, the uncompression script, and tell us if it solves the problem on your machine.  That will help isolate the issue, problem, and solution.  It is not even a bad prop, just to have in your runtime.