Oracle test script fails in execute mode and passes in debug mode

I have a test script(.tst) file which I run in PL/SQL.

When I run the test in non debug mode(By pressing F8), it fails because some fields hadn't been updated when a certain Assert statement runs.

But when I run it in debug mode (By pressing F9 then Ctrl+R) it runs successfully and all fields have the correct value at the check points(Assert statements).

Has anyone any idea why this can happen? It really drives me crazy. It might be something related to parallelism or something else that I am not aware of.