Does Lemon juice change acidic level when frozen?
The acidity, ie. the amount of chemical acid in the juice, will not change when lemon juice is frozen. Acid would need to be neutralized by a base, or in the case of citric acid and other organic acids, very high heat.

Was the lemon juice completely thawed? If not, that could be an issue, all the juice must be liquid for the acid to be free and work on the yolks.

Perhaps there was something else happening in your recipie. If you can edit in basic proportions and method. AS I understand it an icebox pie is mixed and chilled to set, usually no cooking, like to set a custard?