If rendered in bold italic, the Greek character ϑ (U+03D1) and t with a combining tilde below appear as an empty box. Would it be possible to fix this?
U+03D1 is a variant theta--an odd addition to Bold Italic, in which Greek is not supported. Do you mean perhaps theta (U+03B8) in a shape suitable for IPA?
Combining tilde and appropriate anchor added in bold and bold italic. But I won't close this until I know more about the request for U+03D1.
I understand your point about this being an odd addition to Bold Italic. In my field theta or the variant theta are used in transcriptions. The problem occurs when words with the variant theta appear in section headings which are set in bold italics.
Okay, theta1 added to bold italic.