PR 11270 fixes this issue by making such format valid. Preceding the width field by '0' no longer affects the default alignment for strings, i.e. no longer sets the alignment to invalid '=' if it is not specified explicitly.

>>> format('abc', '<8')
'abc     '
>>> format('abc', '8')
'abc     '
>>> format('abc', '<08')
'abc00000'
>>> format('abc', '08')
'abc00000'

This does not contradict the documentation, which specifies this case for numeric types, but not for strings.