Reading the forum, I found this post from another user that gave me a little of insight to my question, when talking about the difference between software and hw transcoding, and why if I disable HW encoding the cropping issues disappear.
and in adittion to that, from plex articles you can find this and I quote
“The output quality of video may be lower, appearing slightly more blurry or blocky. This is especially true and more noticeable when streaming at resolutions below 720p or when using lower bit rate source material. (Hardware-accelerated video encoders are faster, but lower-quality than software encoders.)” . And also “When hardware-accelerated encoding is not available, Plex Media Server will automatically use normal software encoding. Exact capabilities will depend on what you’re using for the hardware-accelerated encoding (e.g. Intel Quick Sync Video vs an NVIDIA GPU).”
I did give a quick read to this article before posting my question because I was looking for the difference between the two toggles buttons and didn’t find it (even though it was there), so I apologize for not reading more thoroughly.
I will leave this as the answer for anyone who had the same doubt as me
