Hey, I hope I'm not too late to join the party... I was having this same issue today, and for me, I think it was a combination of how the MIDI is translated from my main keyboard controller (Korg Triton) to Studio One 4. Somehow the MIDI from the Keyboards master tunning would go all out of whack and cause all kinds of chaos.

My workaround, I change a preset on the Korg Triton and change it back, it would then go back to normal, and Studio One would Play Serum back in tune again. IMHO, I think it's a bug in Studio One 4 than anything else, BTW, I'm using version 4.1.3. Something to do with How the changing of the presets on the Korg Triton cause the MIDI to Sync with Studio One. I also notice when quitting the program and restart things would be fine. But it's still a problem for me, Open a new track and stick Serum on it with the default setting of All Inputs for the MIDI, the tunning will go out. Not a Problem in Reaper or Logic. My workaround is taking an extra step, like every couple hours, before break time to check my Tunings with an app called "LS Tune" But most of the time I can hear it go out of tune. Anyhow, I hope this helps...

 

Here’s the fix:

Clear the tuning file and resave. Tuning file can be cleared by command-click (macOS) or Ctrl-Click (windows) on where it says Tuning File: "(none)" on the Global tab.”

You’ll hear it go back in tune immediately.

From Steve Duda:

“There is a bug in the current release version of Serum where if a nameless tuning file is loaded (or e.g. it was included in a preset) then Serum will keep this tuning file in memory until re-instantiated.

This bug is fixed in the next update (which will be out soon), however even then, any presets saved with that tuning file will still be out of tune themselves (because they are saved in that state).

So in any event it's best to locate the presets which are out of tune and notify the preset provider that they should clear the tuning file and resave.”

Good luck!

 

had the same problem with a korg. if I had the tune/pitch window open I could see how it fluctuated up and down. specially the first half hour after being turned on after that it calmed down, but could act up suddenly. it took me time to figure out what was causing the problem and I manage to f%€k up servers small track ideas at the time.

so I hope OP has already ruled out the keyboard as the problem

 

Had the same problem today, restarting the PC fixed it.