iphone 7 32gb now runs hot at back - ios 14.4.x / apps like discord?

hi, Just reachine out if anyone has a iphone 7 that heats up more than it did?

• iphone 7 now runs hot when running discord app or used for 1 houror more. does not kick off the sensor. apps get super sluggish incl. safari.
• apple store genius this week found nothing from diagnostics incl. no battery sensor issues. he said is due to aluminum back where new backs are glass and scientists would not find anything if we sent it in.
• done a factory reset / backup restore - not tried reset all settings
• just had 5 day followup with apple. told they can replace iphone 7 for C$405.

Anyone else had this problem? Seems a waste to get rid of it and don't think this happened with ios 13