Hi @Vega_may, I do a lot of research on this topic. It can be any reason at all for a shadow ban. If you are comfortable sending me the account details I will look into it when I have time.
Main reasons for hashtag ghosting or shadow ban:
The account is considered spammy in nature or has been reported as being spammy or for other reasons.
Use of newly made "branded" hashtags or banned or semi restricted tags.
Too many tags per post, including other users adding them in comments.
Too many account "mentions" or tagging too many per post or in comments.
Questionable content that may have been reported or otherwise tripped a filter.
While reporting the issue does not do anything immediately, it should force the account or tag to be reviewed by a human at Instagram or Facebook. Having it linked to a legit facebook page should help the issue, some people report unlinking them and then relinking them works (most likely FB trying to force ads for accounts promotion).
Anyhow, as @HenryCooper mentioned, we can't really help since we know nothing about the account or it's history. Even then it's just a trial and error thing sometimes.