Currently it's {'approximate_formats'}. IIRC chaley introduced that function because the {formats} function is/was slow, my problem may have occurred before he did that. It was in my early days of using calibre, perhaps when I was still adding the initial 30+K 'books'. Because it was easily fixed, and hasn't recurred since I assumed it must have been something I did.
@
chaley - if you happen to see this, and you're looking at the approximate_formats code, could you put a space after the commas, i.e. so we get
DOCX, EPUB, ONE rather than
DOCX,EPUB,ONE. IMO it's not a worthy of a bug report
BR