I'm converting a PDF so it has fillable boxes, but when it auto-completes some of the boxes, it detects it as one large field rather than 4 separate fields. I assume it is because the text in the first box is over two lines, compared to the one below where there is only one line of text?
Does anyone know if there is a fix for this as the document is very long and completing it manually is very time consuming.
There's no real automatic solution. You have to spend the time (or hire someone to do it for you) to fix it manually.
The only way to get consistent results when adding fields is to use a script. For example, I've developed this (paid-for) tool that will add text fields over sequences of underscores with 100% accuracy: https://try67.blogspot.com/2020/04/acrobatreader-convert-underscores-to.html