An answer found, posted by raj kotecha on https://answers.microsoft.com/ who writes:
“I encountered the same issue, but managed to work around it by opening the file directly, selecting "File", "Share," then "Embed," and generating the embed code. The problem with src="? seems to happen when you right-click on the file (without opening it) and choose "Embed." That method appears to generate the embed code without including the source. Therefore, you need to open the file first and then generate the embed code.”
The above solution worked for me.
Am curious to find out why OneDrive has made these changes, to Embed code, and whether Microsoft Embed is a stable enough technology service to build functionality on?