Canon EOS 750D (EOS REBEL T6i) Focus Is Unreliable


Camera: Canon EOS 750D (EOS REBEL T6i)
Lens: Canon EF 50MM F1.8 STM
Issue: Focus is unreliable

Settings Used:
Manual
200 shutter F1.8 100 ISO
Correctly exposed every time

This all just started happening, no damage to camera or lens and this is the only lens I can test with it. This lens works fine in Live View on a Canon 100D (SL1).

Situation 1 of 2
I have the above setup and when I use the viewfinder, the pictures come out blurred. However when I press the live view button, and -WITHOUT – doing anything else, then press live view a second time, now when I take a picture though the viewfinder, the pictures come out correctly sharp focused, and I can take pictures of eg, 10 objects and all are correctly focused.

Situation 2 of 2
This exact same issue will happen if I use live view, I switch to viewfinder, and without doing anything else, immediately switch back. The pictures taken will all be sharp

What I have also started to see is a trend:
If I have a sharp focus in either Viewfinder or Live View mode, then press the button to switch from Viewfinder to Live View / Live View to Viewfinder, then the image will be out of focus.

Please can you all help us we are struggling so much.

Thank you in advance.