astropy: astropy.io.fits does not handle CONTINUE keywords in FITS headers correctly

Description

The FITS standard allows header keyword values to be longer than 67 characters by using the CONTINUE convention, see this link. The astropy.io.fits module seems to apply some formatting when doing this for better readability, but under some circumstances does not write the correct content to the fits header and omits information.

Expected behavior

If a keyword value is longer than 67 characters, it should get split in 67 character chunks with an ampersand at the end as the 68th character, and then the value should be continued in the next line with the CONTINUE keyword:

In [1]: from astropy.io import fits
   ...: hdu = fits.PrimaryHDU()
   ...: keyword = "WHATEVER"
   ...: value = "SuperCalibrationParameters_XXXX_YYYY_ZZZZZ_KK_01_02_03)-AAABBBCCC.n.h5 SuperNavigationParameters_XXXX_YYYY_ZZZZZ_KK_01_02_03)-AAABBBCCC.n.xml"
   ...: hdu.header.append((keyword, value, ' '))
   ...: hdu.header
Out[1]:
SIMPLE  =                    T / conforms to FITS standard
BITPIX  =                    8 / array data type
NAXIS   =                    0 / number of array dimensions
EXTEND  =                    T
WHATEVER= 'SuperCalibrationParameters_XXXX_YYYY_ZZZZZ_KK_01_02_03)-AAABBBCCC.n&'
CONTINUE  '.h5 SuperNavigationParameters_XXXX_YYYY_ZZZZZ_KK_01_02_03)-AAABBBCC&'
CONTINUE  'C.n.xml'

Actual behavior

The keyword value seems to get reformatted for the FITS header after a blank in the value, and in the last CONTINUE entry, information from the keyword value gets omitted (the “xml” at the end is missing):

In [1]: from astropy.io import fits
   ...: hdu = fits.PrimaryHDU()
   ...: keyword = "WHATEVER"
   ...: value = "SuperCalibrationParameters_XXXX_YYYY_ZZZZZ_KK_01_02_03)-AAABBBCCC.n.h5 SuperNavigationParameters_XXXX_YYYY_ZZZZZ_KK_01_02_03)-AAABBBCCC.n.xml"
   ...: hdu.header.append((keyword, value, ' '))
   ...: hdu.header
Out[1]:
SIMPLE  =                    T / conforms to FITS standard
BITPIX  =                    8 / array data type
NAXIS   =                    0 / number of array dimensions
EXTEND  =                    T
WHATEVER= 'SuperCalibrationParameters_XXXX_YYYY_ZZZZZ_KK_01_02_03)-AAABBBCCC.n&'
CONTINUE  '.h5 &'
CONTINUE  'SuperNavigationParameters_XXXX_YYYY_ZZZZZ_KK_01_02_03)-AAABBBCCC.n.'

Steps to Reproduce

See above.

System Details

Darwin-20.3.0-x86_64-i386-64bit Python 3.7.9 (default, Aug 31 2020, 07:22:35) [Clang 10.0.0 ] Numpy 1.19.2 astropy 4.0.2 Scipy 1.4.1 Matplotlib 3.3.2