My passport was not stamped in JFK! Help!
I agree that it shouldn't be a problem. It is the same for US citizens visiting other countries and having their passport read electronically but the immigration officer does not give them a stamp. I have not seen an issue.
If questioned, you would just tell the official asking the truth as I imagine even before electronic passport readers, a stamp was forgotten or couldn't be read due to being too light or having smeared ink.