I can't imagine a TVS doing this, it's wired directly across the power. If it were acting like a diode, it would quickly overload the FET voltage control and likely cook them.
If one FET fails, you can get a DC offset, I just fixed a pair of TIU's with that exact problem. I replace all four as you can't really do a test in circuit of the FET, and since I have to remove them anyway, it's easier to chop them out and remove the leads individually. The parts are only a buck, so saving them isn't worth the labor.
