Only can get the bracketed font on those two documents in the font finder of InDesign, yet if I open a new document without closing InDesign or the problem document , the font will appear without brackets and renders properly in the new document. I didn't mention this before, but it is a PostScript font, not TrueType.
Also, I can change the unchangeable Slimbach font with another type family and it will allow the changes.
I don't know how many more old documents I have with Slimbach, but it appears the older documents do not like the font family at all. Both defective documents show the same creation date, even though there were created in the different years, so that shows me that they must be converted PageMake documents with the InDesign plugin. As I said previously, the font shows a foundry creation date of March 6, 1996 date.