Ahhh, I see... the iPad keyboard does not count as a keyboard for keystrokes. So I had to make an OnObjectModify and whenever I need the filter to perform, the user has to hit the return key. Oh well, not as elegant, but it works. I think I remember something about this at Devcon, but it had slipped my mind.
In your script use Commit Records and then a Set Selection to get the cursor back into the field.
That works for FileMaker Pro, but it doesn't work for FileMaker Go. I didn't realize it until one of my users pointed it out. So I had to create the workaround where it will filter when the iPad users hit the enter key. But it sure would be nice if the user interface behaved the same way on Pro as Go. Oh well.
I took the example from FTS-12 Module 9 Script "Commit_Filter_OnModify" and it works fine on my iPhone and iPad. Maybe that will fit your needs?
What about putting a second script trigger on the field. It would be OnObjectExit and the script would do an Exit [False]. I believe that this would keep the cursor in the field when the user presses the Return key. I don't know why I think this, but have a hunch that you may actually need two steps in the script. First do a commit and then the second step of Exit [False]. To get out of the field, they would just click outside of it.
I am not able to test this currently, but give it a try. Hope it doesn't leave you stuck in the field...