Strange, did you try inserting the new field file using cdprog? As long as your file is only slightly larger than the original one it will still fit into the sector.
Mostly that is the safest way to go. Extract your whole FIELD directory using cdmage, then mod the directory using MR and finally insert the modded files in cdprog, that's 100%.
Another issue when using a different method like Hack7 or MassIsoupdate is increasing the ISO size. As I know that you are creating mods you should know that it is pretty tricky to have the image size increased as you can't make small ppfs anymore.
For MR this may be a small bug in the save ISO function.
Yes most definitely a bug IMO. Because as soon as it tries to move it to a new sector the game just doesn't use it, instead it just uses the old script. I never noticed before because I always used MassIsoUpdate. I also replicated the problem with original Japanese version, as soon as it changes the sector for rootmap.dat (this time to LBA 202) it just uses the original script and not the updated one.
So with Makou iso save feature it seems it doesn't like an increase in filesize... to replicate just open the ISO, copy and paste the existing script a few times over in rootmap.dat (until it moves the file to new sector) and now Makou will only see the new script, but the game will default to whatever old script was in the original sector. Now the question is does Makou do this with all fields or just this one?
meesbaker: Yes, it is strange... CDProg is not option due to filesize. And yes MassIsoUpdate increases the ISO size by 2 bytes, but at least it always works and I can insert decent amount of text, which is probably what people using a field editor want to do otherwise they would use translation tools. A lot of people can download like 10 GB in like a minute now anyway.
Edit: Yes it seems this occurs with all fields unfortunately, as I added some very explicit text which did not appear.
Also some other smaller bugs I found:
1. If you add a new text to the end of the list (for example, there's 15 texts, and you add number 16), then you can't use it for a window autosize preview. At least not until you add number 17 (and then 16 works).
2. If you save an ISO and its busy (such as with CDmage) it returns an error about being busy. If you then close CDmage and then try to save it, it will just fail over and over again. So basically you will lose all your work if your ISO is accidentally busy.