Why is my resonance phase lag discontinous?
You seem to have 3 problems
1 - You have a phase wrap. It happens when the phase goes past 180 degrees. The phase will wrap back to -180 degrees. You can fix this by unwrapping the phase.
2 - Your phase is in normalized radians instead of being in radians or degrees. This is not a problem per se but when you try to unwrap the phase, you should be aware that phase unwrap blocks usually expect a phase in radians, not in normalized radians.
3 - The measured phase seems to be inverted compared to what you expect.