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#91
On the LG C2 remote, press ⚙️ → ⋮ → [Picture] → [Aspect Ratio] → try to expriment with the options there.
EDIT: Another option that you could try would be to hook up the Denon to the TV via the eARC feature of the TV. This will let the PC send the signal to the TV, and the TV sends the audio part of the signal to the AVR. (That is, instead of letting the PC send the signal to the AVR, and the AVR sends the signal to the TV.)