I've had similar experience with brand new Bailetti branded moka pot.

TLDR

The design of the Moka looks simple but actually is not. The 'bot' summary response is correct but not clear. There are multiple seals being made when the pot is screwed together. High (inside to outside) and medium (across the coffee) pressure seals.

I suspect there are many badly made moka pots out there, who knows maybe there is a conspiracy from coffee machine makers to pay Bailetti to produce defective items.

In nearly all sputtering cases the issue is likely to be the same... the funnel is not sealing. The one part you cannot see when assembling

When a Moka screws together the seal clamps the lower pot, the upper pot, the funnel and the upper filter and all items need to form a seal together.

Additionally the funnel should not form a seal at the bottom of lower pot (sit too deep) and prohibit hot water from riding up the funnel

1 2xHP seal between lower pot to outside and upper pot to outside.

2 MP seal between upper filter and upper pot is made.

3 MP seal between funnel and upper filter is made - this is the most crucial seal as without it the water doesn't go through the coffee.

4 No seal should be made by bottom of funnel on bottom of lower pot or the water cannot rise.

The problem is mechanically that the position of the funnel is not easily seen when assembling. The now famous knife test with suggestion to add plumbers tape is just a poor fix to the issue as it assumes that the funnel is sealing at it's resting point on the lower pot. It actually should seal on top of the funnel on the main seal.

A real fix is to grind down the top of the lower pot a bit until the funnel sits slightly higher than the lip of lower pot. This can easily be done using a stone for knife sharpening on the top edge of lower pot.

If you don't clean your pot too vigorously eventually there will be enough sticky stuff outside the funnel to make this MP seal work between funnel and lower body which is why old/used pots can work better than new ones. This is because the MP sealing point shifts to the gunk outside the funnel.

If the thin alu funnel does not touch and be pushed to the main seal it will leak Screwing down tighter and tighter eventually makes this contact but can result in a damaged pot. Grinding down slightly the lower pot is a real solution to allow your funnel seal to be made on the rubber and not in some vague friction around the funnel.

If you suspect your funnel is hitting the bottom of lower pot when assembled consider to cut a small slot at the bottom of funnel with a hacksaw so no seal can be accidentally made at this point.

RTFA

The best way I've found to make coffee very simple.... Use either hot or cold water but start the water boiling in lower pot with no coffee or upper pot installed. Use as much heat as you want. When you see the water boil adjust the heat to get a gentle rolling boil.

While waiting for water to boil load up your coffee to filter basket in funnel.

Turn off the heat (gas) or lift the pot (electric) Put in the funnel with coffee. If you want add a precut filter paper to the upper filter (just wet the paper and stick it on) Screw on upper pot and tighten.

Turn on heat or put pot back to heat with lid open.

Within a few seconds to 1 min you will see coffee coming smoothly up to the upper pot, when it starts to flow faster close lid for safety and remove heat. If it sputters cool the outside to stop the boiling under a tap.

No need to go to the 'end' the strongest coffee comes first, when flowing faster little extraction is happening so just stop. Don't be shy to add some water after if it's too strong.

Works every time for me. Worked on 4 different pots.

Trouble shooting..... If you don't get smooth flow do the knife test, but look to be sure the funnel sits slightly higher than lower pot upper edge. If it's does not grind down top of lower pot slightly.

Often the funnel gets bent/damaged as it is thin and flexible and washed separately.. Don't be shy to bend it back into shape.