A Windows PC would have to use Dolby MAT to facilitate the conveyance of Atmos metadata this means that it would be normal to have such metadata output from a PC in conjunction with multichannel PCM.
The AV receiver should still indicate that it is receiving Atmos and processing it even if in conjunction with multichannel PCM. If you are positive that you've an actual speaker setup conducive with what would be required to portray Atmos and that you are sure that no other DSP Programs or SURROUND DECODER modes are engaged then I'd have to suggest that either your PC isn't in fact outputting the Atmos metadata or that your TV isn't correctly configured to pass this through?
Note that in order to be able to convey Atmos using Dolby MAT and with multichannel PCM then your TV and your AV receiver would need to be eARC enabled. Conventional ARC cannot handle multichannel PCM and as such wouldn't allow you to pass this through said TV and out to your AV receiver.
Your AV receiver is eARC enabled. Is your TV eARC enabled and have you configured it accordingly to use eARC as opposed to just ARC? Have you also set it up to BITSTREAM and passthrough audio as opposed to setting the TV to PCM?
If the AV receiver isn't showing Atmos on its display then it isn't processing Atmos. If all the 5.1.2 congured speakers are active even though the receiver doesn't indicate that it is processing Atmos then I'd suggest that you've upmixing in opreration? Set the AV receiver to its STRAIGHT mode to make sure that this isn't the case.
The dealay you are experiencing is the time taken for the handshake between the AV receiver and your TV and for the AV receiver to determine the audio format being sent to it.
I'd suggest this not to be a cable issue. Cables don't effect what audio formats you can access. You either get the audio or you don't and the cable used will not determine the format you get.