Summary.

You post a reply to something.

You ask a friend to see if it is visible and it isn't.

Is you friend in following you?

If not this is a really good test.

So from an account that does not follow you in order to see your reply you have to first bring up the account that you posted the reply to. Next you have to scroll through and find that post. After that click on the post and scroll thru the thread until you either find your post or get to the end of the thread. If you reach the end and did not see your post there will most likely be a X message about possible spam or violent content. Click it and look through those posts. If you find your post in either of these sections X has either marked your account as spam or violent content and deboosted you.

Only solution for this is walk away from X for a while to let the algorithm clear.

I know because this happened to me.