A caption that works on Instagram may be too long for X, and a landscape photo gets cropped in a TikTok feed. Hashtags, mentions and links also behave differently on each platform.
Good cross-posting keeps one idea and adapts its form: the shape of the picture, the length of the text and the call to action for each channel.
Example
A gym's class announcement goes out as a 4:5 photo on Instagram, a vertical video on TikTok and a short text with a link on LinkedIn.