Electronic – Does a “Buck Step Down Voltage Regulator” provide constant output voltage regardless of input voltage

You said you measured the 1.3 Amps across Vin and GND. This is NOT the way to measure the current capability of a power supply - it does give you the short circuit current, but that is usually not a useful value. If the existing power supply claims to be 12 volt and 900 mA, you should believe that current rating, and not attempt to draw more currrent.

To measure current, you must connect your meter in series with the circuit - you break the circuit to do this.

You should measure the actual current drawn by the 12 volt load. If it is less than 900 mA, then the difference is the current you have available to power your step-down converter.

As others have said, the 300 mA rating of the stepdown converter is the maximum it can supply, not what it will actually draw. Since the stepdown ratio of the converter is 5/12, we can expect the current drawn from the 12 volt supply to be a bit more than 5/12 of the load current - perhaps 35 mA. (As a rough approximation, we can assume that the power into the converter equals the power out, plus some losses in the converter.)

The total current drawn from the 12 volt supply will then be whatever you measure for the scale, plus the 35 mA or so for the step-down converter and 5 volt load.

Those regulators are not designed for such high-power applications. You can't connect them serially because they were not made to be poly-phase. If you try to connect them in this manner, the feedback loop in the converter will freak out and can cause very unexpected results which may damage whatever you are trying to connect it to.

Your required large input voltage range poses kind of a problem. I'm having a hard time finding a converter that can accept up to 400V.

Regardless, you are going to need a much more suitable solution. The converters you have are not fit for this in any way. You will want to be looking for buck switching mode power supplies

EDIT: Generally you would convert the 3-phase AC down using a transformer. Common DC voltages output by the transformers are around 24v, 12v, among others and this approach is much more easily usable by other switching mode power supplies.