We recently recorded a gospel quartet with blumlein capture and encountered phase issues at mix. We used a stereo AKG 426 mic and everything was done at once with no overdub in blumlein. I do not believe we had a phase invert accidently engaged on the pre.

I think the phase issue maybe from capture outside a 90 degree span. Could that cause phase issues with blumlein?

Here is a link to the recording.

Mastering for the Amateur