If you generate your font from FL Studio you must check if this option makes any difference:

Thanks, I'll turn if off next time and see what happens (it's on right now).

I've had something like that happen with Fontforge, but likely for a different reason.
To keep my glyph set manageable when editing, I switch encodings often, and set it for Unicode before generating a font. Sometimes I forget myself and leave the Encoding>Compact item checked, and that screws the thing up like you show.

Again, not likely your problem, but I thought it worth mentioning.

It's all worth trying. The cause appears to be so random, the answer will be too.

I've run into problems with 'vanishing glyphs'. What generally fixes it for me is changing the startpoints of the affected glyphs.

André