Are they answering a different point to the one you made?
Behaviour like this is sometimes called
Refuting a weakened or invented version of someone's argument instead of the one they actually made.
Advancing a strong, contentious claim, retreating to a modest and obviously true one when challenged, then returning to the strong one once the challenge passes.
Answering a criticism by pointing at someone else's alleged wrongdoing instead of addressing the criticism made.
What would tell them apart
Replacing the actual point with a weaker or invented version of it, and arguing against that instead, is one shape — restate what you actually said and it either drops the substitution or doubles down on it. Swinging between a bold claim and a modest, easily defended one, retreating to the modest version under pressure and returning to the bold one once the pressure is gone, is a different shape. Leaving your point alone entirely and answering it with an accusation about someone else instead is different again.
Why it might be none of these
People genuinely misread each other's points fairly often, without any intent behind it. The difference tends to show up in what happens after you correct it: a misunderstanding gets dropped once you clarify, while a substituted version keeps getting defended or restated as if you had said it.
None of this covers stalking, doxxing, swatting or harassment campaigns. If any of that is closer to what you're facing, go straight to Red lines instead — the advice on this page was not written for it.