question

That's the whole point of MPPT!!! Maximum power point isn't the same as maximum current! If you measure the voltage when you short circuit the connectors (by checking the current), you'll get VERY close to 0V. Note, when you measure current like you say, you're actually SHORT circuiting, it's not OPEN circuit!... So P=V*I you end up with little power output actually. If you measure the voltage, that's open circuit, thus no current flowing, and even if you get 1000V from your solar panel, and still get 0W in power. MPPT's job is to find the best value in-between, to get the maximum power (current and voltage combined).

In my experience a solar panel will give you 70-80% of its maximum power at most when facing the sun directly. You can increase it by reflecting more sun onto it, or you might temporarily be able to get that with cloud-edge effect (google "cloud-edge effect").

I think that you don't have enough headroom Voltage above the battery Voltage.
Sure, the MPPT will start with 5V above battery Voltage and will run with just 1V above, but it'd be unlikely to get full power, that's Ohm's law at work. The MPPT only has a narrow Voltage range to scan and if the maximum power point that you expect is not in that range, then it won't make maximum power.

How many of these panels do you have? If you have more than one, then connect them in series to get the cell count up and thus PV Voltage higher. This will give the MPPT a much wider Voltage range to scan and find a better power point.

What you have here is an Ebay scam panel. Over stating a solar panels output.

The dimensions of the panel do not reflect normal output.

A Victron 90w panel measures 780 x 668mm (0.52msq)

Your 100w panel measures 530 x 660mm (0.35mspq)

Maybe a 70w panel at most.

Note that the cells are cut (not fully square) and the label is already falling off...

Hat tip to @klim8skeptic

Solar Maths Time!

Solar panel dimensions are easily measurably by most people. So it is hard to cheat on those, but almost everything else is up for manipulation.

I googled the name of your solar panel and got the spec sheet.

In this panel, there are 72 cells. They are connected in 2 parallel strings of 36 cells in series.

Each cell is 125 mm x 62.5 mm - This is called a split cell. You get all the voltage you need (~0.5V per cell), but only half the normal current per cell. Also keeps the panel size down relative to the voltage.

If we convert that to square meters,

0.125 x 0.062

0.00775 square meters per cell.

Solar panels are tested / graded with a light intensity of 1000 W / square meter.

The amount of energy that is converted from light to electricity is called the panel efficiency.

In this case, the panel efficiency is listed at 17%

So we have 1000 × 0.17 × 0.00775 = 1.3175

1000 W × 17% × 0.00775 square meters per cell equals 1.3175 Watt per cell at a theoretical maximum (perfect test conditions)

This panel has 72 cells in total (8 x 9)

That gives us a MAXIMUM possible theoretical output of 1.3175 W x 72 = 94.86 W

So we have already lost the first 5W, just in theory based on their own spec sheet numbers.

It gets a lot worse from there.

If each cell is 125 mm x 62.5 mm, and there are 72 of them in 8 x 9 grid then the panel must be 1000 mm x 562.5 mm MINIMUM (there is also the space between the cells, and the panel frame to add)

But the total panel dimensions are listed as only 630 mm x 540 mm !

This is the only verifiable figure we have, and is probably accurate.

If we use the dimensions of the panel itself for our figures (and this is generous, due to space between cells and frame).

0.63 M x 0.54 M = 0.3402 square meters

Now back to our solar efficiency conversion calculation;

1000 × 17% × 0.34

Gives us a MAXIMUM THEORETICAL output from this panel of 57.8 Watts.

A very far cry from the claimed 100W.

Victron MPPTs have a peak spec 98% conversion efficiency. Lets say there is another 5% lost in wires.

If you are seeing 44W as output from your panels in full sun, the panel is producing about 47W. Which means your real world sun intensity is closer to 813 W / square meter, which is pretty good!

Bottom line, I don't think there is anything wrong with your Victron MPPT. If you purchased the panel from a reputable dealer, I would be taking it back, and testing the next one they give you either by doing some more math above, or even better, a real world test with your MPPT.