So what about the layout itself? Has it ever had a Theme applied?
What are the dimensions of the body part compared to the dimensions of the field using points as a measurement?
The reason I asked for the size in points is that I have found anything which is not a precise point may render incorrectly, its easy to get to this state when using other measurements and working with grouped objects.
Also, are you using any scaling, i.e zoom level or 'enlarge window contents to improve readability"? This can have that effect too.
I use a similar technique in portal records to shade every 3rd record, although I use condiitonal formatting to set the colour and have a bottom border instead of a top one, because of the decimal precision and my need to have the object edge to edge, its been a bit of a pain in 12. Ive not gotten to list layouts yet.
Both "Rates" and "Accounts" are listed as having a Cool Gray theme, but I assume that's because it's the default. What actually appears on the page looks nothing at all like Cool Gray. It looks like what I created using the then-default settings (mainly Helvetica 12) in FMP 8 or 9 back in the middle oh-ohs.
The body part is height 81, beginning at 112. The "Bkg" field is height 80, beginning at 113. (I habitually prefer points as my unit of measure and tend to operate in multiples of 6. Thus the top edge of actual usable fields in my body part is at 114.)
Every object on my layout is sized and positioned at an integral number of points.
No scaling. (Well, the images I posted were at 50%, but you can see the overline in the top image but not in the bottom one, so that's not the problem.)
Themes have extra pixels that you can only shed by modifying the Theme (which is not advised by FMI).
I'd say you need to have the Bkg at maybe 114 or even 115.
There is a frame border and object borders that we have no control over. I think if you use the CLassic Theme you will get the right result. Mind you, I'd create a new layout and apply no theme (or default themes) and paste.
I am definitely NOT going to be fiddling with the specs of any of the themes.
Believe me, I tried turning the overline on and off, applying a thicker line, changing the color, applying it to the bottom and sides as well as the top, moving the "Bkg" field to the front, moving it to the back, changing its size and position within the body part — all of the variables I could think of. Results were the same every time: the blue always showed up, the overline (or whatever I was trying instead) never did. That's why I figured it must be something that FMP 12 had added that I wasn't aware of, that only affected newly created layouts.
Update: On the theory that maybe there was something glitchy about the field I had copied, I used the field tool to create a brand-new instance of the "Bkg" field on my layout. I removed all styles from it (necessary to get rid of the internal padding the field came equipped with, if for no other reason), then tried again to apply the overline. Once again the overline DOES show up on the active record but not on any of the others. The blue shows up where it's supposed to in every 5th record. So it doesn't seem to be a problem related to copying and pasting the field or to the conversion from the .fp7 format, it's something that FMP 12 is refusing to do de novo.