Not a bug. Lots of ways to do things in Samp, it's got to be something that was overlooked. I assume you are working in a virtual project instead of doing destructive editing, which means the effect is applied on playback or when writing the export file instead of directly to the original file. Sounds like it is not exporting the stretch effected version. Double check your file export settings to make sure it is selecting everything you want to export, including effects, maybe try track bounce instead of export, etc. I'm not in front of a Samp machine right now so I can't offer specifics, but I'm sure it's an overlooked checkbox or something.