Phase of matched filter for chirp signals
For a constant frequency signal, with no frequency offset between transmitter and receiver, the phase of a complex correlation, at the correlation peak, is the phase difference between the received signal and the reference signal used for the matched filter.  It need not be 0.  And that is for the actual correlation peak, which may have to be obtained by interpolation between the two samples that surround the peak.
I haven't worked out the math for a chirp.  I would not rely on the phase of the complex correlation being 0 at the correlation peak.