My work around was to copy and paste the textbox as an image, crop it, and then save it all as a JPEG, which worked fine.
Interestingly, next day, when I reopened the original Visio, the text box selection border was much larger than originally displayed, extending beyond the yellow 6Wx6H rectangle, but I was able to select all and save as a JPEG, at identical 6Wx6H sizes.
So my conclusion is that I was encountering a bug of some kind, although I wasn't able to immediately replicate it. Challenge for another day.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…