If the correct answer to: "I didn't want to listen for fear that you'd be lying." is "Je ne voulais pas écouter de peur que tu ne mentes.", then this seems to be somewhat in conflict with another question "I didn't do it for fear that you would be disappointed." whose correct response has been noted to be "Je ne l'ai pas fait de crainte que vous ne soyez déçus".
The confusion (at least to me, is the use of the verb "to be" in the two sentences. If the correct answer is not "Je ne voulais pas écouter de peur que tu ne sois mentir" then perhaps a better phrasing of the English sentence would be: "I didn't want to lister for fear that you'd lie" thereby bringing emphasis to the verb "to lie" and away from the verb "to be". Of course, there is always a distinct chance I'm totally missing the point.