I use ArcMap 10.6.3 and I've been using the join field tool to add large amounts of data successfully from excel spreadsheets to the attribute tables.
I would add the spreadsheet to the map and export it as a Dbase table in order to add OIDs so I could use the join field tool.
I do this by a common field of building number, however recently it says the tool has been successful even though it only adds a small chunk of data, specifically only buildings with numbers 999 and under.
I have no idea why this is happening so I though I'd throw it out to the community to see if anyone has any theories!
Use Excel to Table, into a file geodatabase. The excel table presumably has an equivalent join field of the same type as the destination and there are no blank entries?