Fillable PDF

I had similar issues, the lines need to be sometimes darker to be picked up more accurately, sometimes, I end up adding them by hand (Acrobat tool) because I was fed up of trying different options.

I go tthe best results when I created the fields in Word or Excel with a gap between each. I made sure there was a bit of 'empty space' between each so the automatic detection worked without being confused, merging cells or missing some.

I now use InDesign for that reason

It's not an error, per se. The fields detection wizard is far from perfect and its result can vary widely.

If you want a consistent way of adding fields it has to be using a script, and even then it will probably require some manual tweaking.

I've developed a (paid-for) tool that can create text fields over any sequence of more than one underscore characters, and it works with 100% accuracy. You can find it here: https://try67.blogspot.com/2020/04/acrobatreader-convert-underscores-to.html

I've also created a tool that converts certain Wingdings characters into check-boxes or radio-buttons. See: http://try67.blogspot.com/2015/09/acrobatreader-convert-wingdings-to.html

Using these tools it's possible to get a pretty good head start on your form.