the ANSI escape sequences for screen erasing start with ESC and end in J. When you display a string on a screen including a newline at the end of it, you need to erase to the end of the line on the screen so you don't leave predecessor garbage which would be confusing.

That's how the J got inserted, then whatever is processing this is swallowing the rest of the escape sequence but leaving the J. The fix is there, take the J with the rest of the escape sequence at whatever layer is doing this.

  ESC[J   erase in display (same as ESC[0J)
  ESC[0J  erase from cursor until end of screen
  ESC[1J  erase from cursor to beginning of screen
  ESC[2J  erase entire screen
  ESC[3J  erase saved lines

ASCII and then ANSI were invented as in-band communications and terminal control protocols, not "file formats". You thread filters together like beads on a string, where ASCII is somebody's job, and ANSI is somebody's job, and then the rendered text is presented to the user.

unix is ingenious how it has accomodated changes and conflicting ideas, incorporating the new squoze next to the old, like UTF-8 because the same people did that. It's not perfect, but it proved "worse is better" because it for a long time it outcompeted its alternatives and never became bloated and overburdened by bureaucratic committee ideas.

however today's developers did not "come up" in the same culture of apprenticeship learning so a lot of the clever subtlety has been lost and now worse is starting to be worse, and guess what, there is no better.