Newly branded hashtags can be sent to some kind of SPAM filter. Until it gets X uses, or used X number of times by other people, it can have adverse effects on your account. If your new tag is too close to an actual banned tag, the same thing will happen, but it’s a bit harder to shake.
Broken, banned, or incorrect hashtags will do that. I’ve have had a post shadow banned by someone creating one as a joke about one of their friends names on a meme account I had. Result was instant hashtag ghosting. I deleted the post and everything was fine after.
If you also have a certain type of account, or enough numbers or reports by other users, it can have the effect on your account.
In short, there are a ton of reasons why it could be happening.
Also, how do you know you are shadow banned? How have you been checking?