Something is causing the current portal row to lose its "focus".
Commit records is a common cause of that if you have that in your script. Other events, such as a layout change can also do this and if another script trigger is being tripped, a second script might be interfering with the first...
I guess you're right. I've tried the set field to "" and seems to be working, although there's a slight lag when i use the layout on webdirect. The clear field was much snappier.
Both Clear and set field require that the focus be correctly set on the portal row in order to clear a field of the correct row.