Kit lens – Dark manual mode in "Image mode" and normal in "Recording mode" [Canon T5i]

I took some pictures with my Canon T5i + Rokinon 35mm T1.5 Cinema and it was normal. Then, I switched to the lenses of my 18-55mm kit (which is an automatic lens in terms of aperture). Then, I went back to my Rokinon lens.

The problem is that when I take pictures in Manual mode, the live view is too dark, but when I see the image later it is as it was supposed to (much brighter). But when I'm recording, the light is normal. The LCD shows it the same as when I see the picture after taking it.

The main problem is that when I am about to take the picture, I can hardly see anything, even if I set the highest ISO and the shutter speed (with 1.5 aperture). It just started happening now, before it really worked well.

PS: In AV and TV mode, the brightness is as normal as when I use the Manual mode to record.

PS 2: When I am using the kit lenses, the light in the Image and Recording mode (in the Manual Mode) is the same, as it was before with the Rokinon.

So, what could have happened? What can I do to solve this?

Thanks in advance!