All the digital attenuators from Aeroflex mentioned in the datasheet that, "phase-free" (phase invariant) is an option for them.
Did you ask Aeroflex for this option?
From my experience even a very low phase variation digital attenuator using PIN diodes, will have something like +/- 2 deg variation, and this at lower attenuation range. When attenuation is higher, the phase shift increase.
Beware that the phase shift also varies with frequency, and when I say frequency, I mean very small frequency variations.