Cyrillic fonts lost in PDF

My document has a number of Cyrillic fonts that were successfully reproduced in a low-resolution PDF — until today. Now they are fine in the original InDesign document but showing as little X'd boxes in the PDF. I tried saving the PDF in a high-resolution format and that worked — once. Now it doesn't . The font is Adobe Garamond Premier Pro Medium, which DOES have these fonts in its family. I'll have to try Times New Roman as a substitute, I guess. Has anyone else had this problem? Could it be because in desperation I am using Acrobat 2020 instead of 2024, so scroll bars are accessible? If so, another it's another Adobe problem they have to answer for.