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Let's say that you have a bridge rectifier feeding a regulator directly, and the filter capacitors are open.

Whenever the input voltage rises sufficiently for the regulator to work, the regulator output flat tops at the desired voltage.

When the voltage drops below the part where the regulator works, you're likely to get a volt or so less than the immediate voltage to the regulator.

Looking at this with a scope, you'd get a lot of ripple on the output of the regulator.

If you were feeding the regulator with half wave rather than full (say one diode was out), then you'd get worse ripple.

I'd look at the input to the regulator, and the output from the regulator to see what's going on.

Since in this scope, the supplies are likely referenced to only one supply, I'd make sure that the supply that is the reference is perfect.? Ripple on other supplies will not really affect the reference supply, just the individual supplies themselves.

Since the other supplies use the reference supply, any faults on the reference supply are likely to be reflected in the other supplies, even if the DC voltages to them are perfect, and the regulators on the other supplies are working perfectly.

You might want to consider what your meter reads on the DC or AC range when there's DC riding on AC, or AC riding on DC (depending on what you wanted to measure).

I'd really look at this with a scope to see what's what.? Then adjust your perception of what the meter readings ought to be.

Harvey