Upon defining a style with its only attributes set to “Bold”, the “Style Settings” box still reports its attributes as “[None]” (without stating that it’s actually “[None] + Bold]”). The “Basic Characters Format” still states it to be “Bold”.
I encountered this as a nested-style which relies on a “Bold” style stopped setting its Bold where necessary. The document was a previous-version-created document, and I began a fresh document re-defining all styles. The unreported-Bold attribute behaviour is evident in the newly created v13.1 InDesign document.
Version: InDesign CC 2018 v13.1 x64