After exporting a font as TrueType, the Unicode values and advance widths were shifted by one glyph. For example, the e has the Unicode value and the advance width of the f.
Is this a bug in Glyphs or something specific to my file?
This happens from time to time if the source file is not properly sorted. If you update the glyph info for one glyph, it will sort the hole font and fix the problem.