Tubes microphonic?

Pardon my ignorance on this topic. I’ve encountered microphonic tubes in guitar amps before and am aware of the tap test, but curious if the following is normal behavior.

I was mixing into my ZOD DI’s today and noticed more harmonic content than usual. I was confused what changed since last time or if my ears were just playing tricks on me. I decided to mute my monitors and throw some headphones on and in that moment noticed I could hear the program material emanating acoustically from the DI (I couldn’t tell if it was coming from just one or both ZOD’s, but in any case one sounded louder than the other.)

Is this normal behavior that I just never knew about? Or is it possible the tubes have gone microphonic? Anything that would cause this to happen suddenly?

I’m going to do more testing but thought I’d get a quick gut check from you kind folks in case this is just normal behavior that I never realized before.