Recently, I did come across that we can't able to create an Email performance report under the A/B test program with Subject line test variant.
So, we did analyzed this and found what causing this issue in Marketo. When we clone any existing A/B test setup, then the email performance report won't be available(can't able to select test variants) for that program, until remove the existing setup and re-built it again. So, if anybody clone the A/B test setup further, then please remove the existing setup(Click=> Remove A/B test option in control panel) and built it newly. So, that the Email performance report will be available for the emails that used on the program.
Please remove the existing A/B test setup on cloned programs and re-create the same for avoiding these kind of issues.
Also, raised a ticket with Marketo support to confirm whether it is an expected behavior or not. Will keep you posted the updates
Right, this is known behavior (but understandably confusing!).
You must (re)create the test and approve the program before the email variants will be available in the EPR.
Thanks for checking and confirming this!