Hi- I had exactly the same problem, using Sky Q, I emailed Hisense support and they replied as below:
Dear Hisense Customer,
Thank you for your email and explanation of the current issue you are facing.
I can inform you that the aspect ratio is generally locked on 2018 models and above if the content is 4k.
I have been unable to find the technical reason as to why this is, but the support team cannot change this feature and it is not considered to be a fault.
I hope that this will be helpful.
Please note you can also call our call-centre on 0333 800 2200 quoting reference 1 during these hours:
- Monday to Saturday: 9am-6pm
You can also use this handy link to chat to us now:
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For further help and assistance with your TV please visit our digital self-help guides by clicking this
link and searching for your model in the top right corner.
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Kind Regards,