When a signal has an interharmonic, is the signal periodic or non-periodic?
Let’s say you have an interharmonic of 1.5.  If you scale your assumed fundamental frequency by 0.5 then you will have two integer harmonic sinusoids contributing to the signal - a 2nd harmonic and a 3rd harmonic.  Equivalent, of course, to the original signal - just an integer way to look at it.
First case:  Fundamental = f1
$$ x(t) = \cos{(1*2 \pi f_1 t + \theta_n)} + \cos{(1.5*2 \pi f_1t + \theta_m )} $$
Second case:  Fundamental = f2 = 0.5f1
$$ x(t) = \cos{(2*2 \pi f_2 t + \theta_n)} + \cos{(3*2 \pi f_2t + \theta_m)} $$
These are equivalent signals (and periodic), I just chose to assume a different fundamental frequency for my analysis so i would have integer harmonics.