The monetization icon is applied automatically (via machine learning technology). If you think that there's a mistake, you can ask for a review here: https://yt.be/help/NW2d . You can also upload it as unlisted to get an early read on the status before publishing.
The automated system scans your video multiple times before it’s live & continues to analyze new info once it’s published – this means the icon may change (esp. in the first few hours after upload). We regularly push system updates to improve accuracy, so hope it improves soon!