I had this issue last year and it turned out to be a bug. See below the response to it from @connor which worked for me:
"Thank you once again for the detail of your report. In a significant development, the team have actually been able to reproduce this issue when we use the Procedural EQ with an embedded Parametric EQ as a rule (ie. the original conditions from your report) and play back Mono mp3 content like the file you sent. It’s an issue within Roon’s Procedural EQ, as far as we can tell.
In at least one case, reproducing had far more serious consequences for playback and required creating a new RAATServer instance. I would recommend taking this RAATServer refresh step in your case as well, just as a precaution.
You can generate a new RAATServer instance on your device by following these instructions, but please be aware that this will reset your Roon Settings → Audio Tab to factory settings and I would advise making a backup of any custom DSP settings you have (other than, of course, the Procedural EQ rule that doesn’t work 
):
- Create a Backup of your current Roon database
- Exit out of Roon
- Navigate to your Roon’s Database Location
- Find the folder that says “RAATServer”
- Rename the “RAATServer” folder to “RAATServer_old”
- Restart the Roon App to generate a new RAATServer folder
The team will investigate the issue with multiple filters in Parametric EQ in Procedural EQ rules earnestly with a dedicated ticket. Thank you for bringing it to our attention.
Fair question. Now that we’ve identified the issue on our end with the Procedure EQ → embedded Parametric EQ, please disregard my earlier instructions regarding mixing, channel mapping etc.
In the meantime, I recommend keeping the “Downmix as needed” setting applied in the Device Setup and recreating these notch filters in the standalone Parametric EQ filter. You shouldn’t have any issues playing back Mono files if the rule isn’t embedded in the Procedural EQ."