I use also JOSM 9979. I just retried adding a source=Bing and try to upload. The Data Validator complains about a misspelled value saying "Value 'Bing' for key 'source' looks like 'bing'. - Misspelled property value (1)"

Do You use the Data Validator?

Hi,
JOSM recommends to change it from Bing to bing. So I accepted the change.