If you don't have an HDCP 2.2 display, then the output will revert to hdcp 1.4 with a max 1080p output. This is legitimate and many people use this to get Atmos on their 1080p system.
As for the other path, it can vary. If Oppo is working properly, it should be reporting to the upstream device that the pathway is HD only since the maximum HDCP supported is 1.4. Of course, it also depends on the comcast box knowing what to do with this information and set the output appropriately. Of course, it could be the comcast box is doing the right thing and outputting 1080p, just when it checked to see if you could use 4k, it did an incorrect check and just relied on whether it could use HDCP 2.2 on the current link, not whether the device downstream supported a full 4k path.
It could also be the Oppo is working incorrectly and only encrypting on a link by link path and not passing the HDCP keys upstream. This also happens often enough because it is quite complex handling multiple repeating devices. HDMI splitters almost universally get it wrong which is why you can often use them to remove HDCP.