The documentation for the Play Ringtone action ends with:
Note: in some Android versions ringtones will endlessly loop due to an Android bug.
Tasker can't supersede android bugs, so you need to abandon Play Ringtone and use different actions to achieve the same result.
Try Music Play and point it to the same ringtone. If your built in ringtones are inaccessable, look for your ringtone on the net as an audio file, download, and place in you phone and navigate to the folder you decided to put the audio file.
The play allows you to loop or only play once, if you do choose loop, then Music Stop will silence it whenever you want, after a specified Wait time, after a condition is met, after a Variable is matched, etc.
Hope this helps,
Logan