This may happen for OP-1A MXF files, particularly XDCAM or other format with variable frame size, due to a tailing frames indexes not being read correctly by Resolve. This bug will be fixed.
However if that happens to DNxHD which is constant frame size, this looks odd, thus I’d appreciate if you could provide us with more details (is it an OP-1A or OP-Atom MXF, FrameRate etc) or a file sample.
Best regards,
Konstantin