The Bull Rush skill, etc, should be added to the first available slot, even if you add or remove items, or unlock the skillbar and rearrange the skills.
Does a save and reload fix this when it happens?
Please email supportdos2@larian.com, with a description (or link to this topic) and a save where this has happened (or a Dropbox or Google drive link, etc), or before if you notice any situation where this is reproducible (sounds fairly random).
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2\PlayerProfiles\<ProfileName>\Savegames' folder. In Windows Explorer you can zip file(s) and folder(s) by right clicking on it/them and selecting 'Send To | Compressed (zipped) Folder'.