The group that released this specific encode. They are known for small file sizes and specific "hardcoded" metadata that requires matching subtitles.

Move the subtitle file into the same folder as your movie file. Rename the subtitle file so it is identical to the movie file name (excluding the extension). Movie: The.Predator.2018.720p.BluRay.x264.mp4 Subtitle: The.Predator.2018.720p.BluRay.x264.srt

The source material. BluRay rips usually have a frame rate of 23.976 fps; subtitles for "DVD" or "HDTV" sources will likely be out of sync.