This seems to be a known issue in drupal, personal commentary redacted.
So I set up a date format, and any non date-format characters get reprinted at the end of the date.
I first had my formatter set up as
Y年m月, but read in the issue that I need to escape characters, so:
Preview in Date and Time preview area:
Looking in views where the field is being output (to json in case that makes a difference), there is no custom option for the formatter, so I couldn't try recreating the date formatter in views to see if that would work better.