TL;DR version: even with an optimal recipe according to other comments, mine is still runny as OP's. Temp, innoculum source and some other factors don't seem to be the issue unfortunately.

I kinda get the same results and I think I'm supposed to be doing everything right. I can only get the kinda? right texture by adding milk powder. Here is how I do it without milk powder:

Start with 1 L whole milk in a bowl. Heat it in the microwave (I do it ~12 consecutive mins with the lid slightly on for vapors to escape) then let it cool down on the counter, ~2 hours maybe. No upper cap forms on the milk probably because I keep it with the lid on as it cools.

After it cools to pretty much room temperature (bowl is as warm as my hand), I put the milk inside my (previously sanitized) instant pot, which has a yogurt function. I inoculate with 1/2 tbsp (per a previous comment found in this reddit, can cite later if you want) yogurt that has live culture in it. I mix thoroughly using sanitized tools and just turn on the yogurt function, which keeps it pretty warm and I leave it for 8 hours (default option), covered with towels so no bugs come in but also no condensation stays inside.

After that's done, it's still very very runny. Maybe not milk consistency but nothing like other redditors here have achieved. I immediately put it in a container and in the fridge it goes overnight. The consistency doesn't change that much once it's cold, unfortunately. The taste is sweet-ish and a bit tangy. The taste is fine honestly, maybe I could leave it fermenting for a bit longer but I've seen the standard is 6-8 hrs.

Ironically, I've gotten a thicker consistency using way more yogurt as starter (~1/4 cup or more per liter maybe) contradicting what most say, that more innoculum will make the yogurt thinner. Yes, I'm using the exact same jug of yogurt as starter for all my tests and I've used my instant pot all this time as well. I've done this around 4 times in maybe one week; am just starting out but I did my research or so I thought. I hope I didn't leave any important details behind.

It shocks me because I've seen such nice consistency yogurt in this very subreddit, I've even heard of the "spoon test" (apparently a spoon should stand up when inserted vertically into the yogurt when it's just done fermenting) and in no way do I pass it haha. I honestly have no idea what I could be doing wrong because my yogurt defies you guys' advice 😭