According to the ovf specification [1] section 5.1:
"The manifest file shall contain SHA digests for all distinct files referenced in the References element of the OVF descriptor and for no other files."
The problem is that ovftool includes SHA digest for the ovf file itself which is of course not referenced in the References element within itself. I think ovftool should behave according to the specification and remove calculation of this digest.
[1] https://www.dmtf.org/sites/default/files/standards/documents/DSP0243_2.1.1.pdf