As per this thread at forums:
https://community.adobe.com/t5/illustrator-discussions/every-line-composer-bug-text-shifts-when-moving-text-frame/m-p/13992952

Ai will recalculate justification each time a text frame with Every-line Paragraph Composer is nudged, either with arrow keys or dragging.

Although it’s clearly just moving, and should not trigger recalculation, Ai does it (I can think only of one case when it should happen — when objects with Text Wrap enabled are present and intersecting the the moved object — and they aren’t and don’t).

The text wobbles a bit and it's slow, to recalculate this every time.
Why does it happen? InDesign does not do this.