Left Channel Distortion throught RPI Headphone Jack

I tried Volumio over the weekend and had a persistent issue with distortion in the left channel that sounded like clipping in one channel when the Volumio “software” volume was set to 100%. The distortion was not present when the Volumio “software” volume was set to 85% or lower. I am only using the headphone jack on the RPI at this point (i am still undecided on whether I should purchase a DAC “hat” or purchase an outboard DAC like the Schitt Modi). I adjusted the settings in Volumio, but the adjustments did not eliminate the issue. I plugged the RPI into another electrical outlet to rule out a ground loop. I plugged headphones and a mini integrated amp into the RPI, but the problem persisted. To rule out a hardware issue with the RPI, I installed the raspberry pi operating system headless onto an sd card and connected an external hard drive with flac files to the RPI’s usb port. I played those files through VLC music player using the RPI’s headphone jack and the distortion issues were not present, even when the VLC software volume was set to 100% Therefore, the distortion appears to be a Volumio software issue or a settings issue. Anything I might try or is the distortion from the RPI headphone jack to be expected?

Can confirm that, for some reason, the headphone mixer allows up to 4dB gain, too much in my opinion.
Everything over 85 (dB gain -0.00) leads to distortion and not just on the left channel.
Not sure if anyone seriously uses headphone out for our purposes, there are already much better results to be had with a simple USB or HAT DAC.

Thanks. Appreciated.