Oddly, when taking a photo of the Nikon D7500 which has an Eye-Start Sensor, my camera captures a red-purplish glow on the sensor while it is completely invisible to my eye. Take a look at the sensor right above the viewfinder:
No matter how closely I look with my naked eyes, even in darkness, I cannot see the dot in the Eye-Start Sensor yet my camera captures it every time, even under bright studio lighting.
I do not want to add an IR-cut filter to the lens because of impact to image quality and flare, put not all my lenses support filters and sometimes I use polarizers or ND filters which I would have to stack.
If you only see the effect when the D7500 is turned on but not when the camera is turned off you are almost certainly seeing near-infrared light recorded by the camera taking the photo.
It's fairly common for most digital cameras to record such light. The easiest example I can think of are the near countless photos people have posted in various forums of an infrared remote control for their television pointed at the camera with one of the buttons pushed when the photo was taken.
(If you also saw it when the D7500 were turned off it is possible it could have been a specular highlight from your lighting reflecting off the exterior coatings on the Eye-Start sensor - but that does not seem to be the case since you don't see it if the D7500 is turned off.)
Run this test: Image your IR TV remote while pressing the volume up button. Can you image the IR LED outputting IR?
Is my camera capturing infrared?
Yes. All cameras are capturing some infrared. Digital cameras are particularly good at capturing IR, that's why most of them have internal filter that dims out the IR.
Can this be fixed internally?
No. It cannot be "fixed" at all, because it's not considered a defect. The camera likely already has internal IR filter, but it's not ideal (nothing is). You can try external filter, but it either won't help (same as the internal one) or degrade lower portion of visible spectrum (filter out visible red light).
I believe this is the IR sensor that detects your eye (or most other things) near the viewfinder so it can switch off the display screen for easier viewing. Without this, your eye when it is on the viewfinder, would get the glare from the screen below. I am a Canon guy, but I think this is the same function on mine as well.