This bug seems to be sequence related. Could be becuase a line of code which sometimes is executed override existing set conditions.

I'd double check on all occurances in the code for:

1. 'set class lib' make sure they have the word 'additive'.
2. 'set path' commands that could overwrite existing conditions.
3. All commands that may change the current drive/folder such as 'set default to' or 'cd'.

Hope that helps.