The way i have got around this is by adding your own icons in a different way

I managed to add my own custom Icons to the Jira prioirties that clearly displayed P1, P2 P3

Sorry for not adding Visuals in this walkthrough, it wouldnt allow me to attach them. if you follow the steps below though you should end up with the same outcome that i did. 

• So what i did was, create a new icon in, Paint, photoshop or gimp, you choose your software then i saved the file as a PNG.
  • (Alternatively or find an approriate PNG image online).

• Then go to a project you currently have in your projects tab.
• Go to project settings (located on the left of your project board screen at the bottom)
• Where it says avatar in the middle of the screen select the "Select Image" option to the right of the current Avatar Icon.
• Select Upload New Avatar.
• Select the image you want to add to your priorities.
• Using the resizer make sure your image fits within the highlighted square using the transformation markers on the corners.
• Once you are happy select the "Confirm" Option.
• Now you have your image is hosted in a location you can access with in Jira and you will be able to get its image address.
• On the Avatars page, right click you image and select Copy Image Address.
• Go to the settings wheel in the top right and select the issue option from the dropdown.
• Scroll down the left menu until you see priorities (under "Issue Attributes") and select it.
• Now if you want to create a new Prioirty level use the Add Priority level fields at the bottom.
• Alternatively if you want to amend a current priority select Edit on the Priority you would like to change the icon on.
• Now you want to use the image address you copied earlier in the Icon URL field, Just paste it in and when you hit Update or Add (if you are creating a new prioirty level) you will have your own custom icon for that priority.

This is how i managed to get around this issue in the past and add my own custom icons that will show on tickets in your priorities field.
You can upload most images in this way to your priority fields and use them through Jira.

Hope this helps.

As a product manager, I need an icon that clearly indicates that the priority level is up for discussion, rather than the default "Unknown" which looks like we haven't thought about it yet. I'd like to add a green question mark but the instructions are confusing and the icon doesn't show up even when following the instructions. It would save me time if this were built in, and help disambiguate priorities.

https://getsupport.atlassian.com/browse/JST-855445

Please add this, I have had so many clients ask me to customize the icons and the documented process always fails. 

Would love to have this functionality, we also need P0-P4 icons. 

Totally agree with this recommendation

This was one of my biggest disappointments in my transition from server to cloud. We need P0 to P4 icons, the pictures for priorities displayed right now do not make sense to our team

I'd like to add some custom icons.

The current icons do not make sense to our org. We need P0-P4 displayed!

Can't believe this isn't a standard and consistent capability across Jira, I've just hit the same brick wall myself !!