The bandlimited solutions require two extra oscillators. The frequency ratio of these vary depending on the number of target partials to synthesize as well as the c:m ratio.
There is certainly the possibility that floating point rounding error leads to the observed drift. The thing that is a bit weird is that the drift doesn’t seem to continue. Once the higher partials are no longer suppressed, there is no further drift back to suppression—which is what I’d expect if that really is what is happening.