I don't see why this would make a difference in how your scripts work, but I'd use Set Field [Issue:: DateClosed ; "" ] to clear the field.
Try your script on a new layout where you've placed just the fields listed in your script and see if it works there. There may be an issue with your layout that's interfering.
I copied the script and changed the two Clear Field functions to Set Field, as you recommended. No change.
I created a new layout from the same table with all fields and applied the same field settings to the three fields in the script (Status to the drop-down list and the other two to calendars). I tried both scripts, and the exact same result.
I created a new layout from the same table with only the three fields in the script (set Status to the drop-down list and the other two to calendars). I tried both scripts, and the exact same result.
In Manage Database/Fields the only settings for the three fields are Indexed. I tried all variations of Validation, but no change.
Just to make me crazy, I created an entirely new database with one table and just those three fields and set them the same way. I imported the two scripts, tried both, and no change.
Then I deleted the imported scripts and recreated them new. No change.
Try running it from a button or the script menu. I suspect the triggering record save event may be an issue here.
It was the script trigger. I thought that earlier I had tried OnObjectModify, but I had been setting it to OnObjectSave.
I went back to OnObjectModify, and it works!
If you end your script with Exit Script [False], it should work with OnRecordCommit if I read the help file correctly. In triggers where the trigger fires before the event, this step will cancel the triggering event.