My goodness, why do you suppose they did that? Clearly there wasn't a building inspector involved!

If I lived there, I would find a way to make the deck surface all one level. Raising the lower part will interfere with the stair spacing, so the steps would probably need to be replaced as well.

How big is the deck? What do you use it for? Is it possible to add a railing or furniture to block the step, at least on one side?

there are 3 parts to this deck and its a huge deck. They had a smaller one which is not in the picture and I suppose the lower one to have access to the yard, garage etc. The middle one ( i.e. the upper part of the step) was either already there but old material or non existent but its the newest. I think the contractor they hired didn't advice or it must have been extra $$ to fix the level issues so they let the levels be off.

My kids play on the deck in summer but even if its safe overall this part worries me and I have to hover while they are there. I like your furniture idea quite a bit and might explore that. I might add some type of weather proof tape for the deck to note any section that we will need to leave open for access to the stairs.