The problem is that the positioning instructions in the PGS stream are made for the uncropped video dimensions.
You can instruct the player to scale the subtitles differently, with
--image-subs-video-resolution=yes
in the mpv.conf, but the results will be sub-optimal.
You will also probably see that the size and position of the subs will change, as you change the size and aspect ratio of the player window.
IMHO, your best course of action is to send the subs through an OCR process to generate an SRT version of it. Introduction: convert image-based subtitles to SRT with Subtitle Edit
