Why does my new front loader leave soapy residue on towels?

Yeah, eyeballing detergent like you did with the top loader is almost certainly the problem. Front loaders use so little water that the same pour you were used to leaves way too much residue behind, and towels soak it all up. The stiff, slightly soapy film you're describing is classic overdosing, especially on towels since they're thick and hold onto suds like crazy. Go by the lines on the cap, not the instructions on the box or bottle. Those instructions assume an old top loader. For a normal load in an HE front loader, line one is usually plenty, and honestly a lot of people go even lower than that, like half of line one, and get better results. Towels especially. Use less than feels right and see what happens. If you've got soft water you can cut back further, but with hard water you'll want to stay around line one rather than dropping too low, since the minerals eat up some of the detergent's cleaning power. And yes, hard water absolutely matters. It makes detergent less effective and leaves its own mineral film on fabric, which adds to the stiffness. Two easy fixes: use a bit more detergent than you would with soft water (but still way less than the bottle says), or add a scoop of washing soda or a dishwasher detergent tab with some loads to soften the water in the drum. Regular cleaning vinegar in the rinse cycle helps too, though don't mix that habit with bleach loads.

The extra rinse option is worth using in the meantime while you dial in the dose. It does use more water, sure, but it's a band-aid, not the fix. Once you get the detergent amount right, you shouldn't need it for regular loads. Might still run it on towels anyway just because they hold residue worse than anything else. One more thing: run a cleaning cycle or a hot wash with a washer cleaner tablet soon, because if you've been overdosing for a couple months there's probably buildup in the drum and the gasket. And leave the door cracked after loads so everything dries out.

Same thing happened to me when I downsized from an old agitator machine. I kept using roughly half a cap for months and my towels still felt waxy til I dropped to about a teaspoon's worth, basicly a quick pour straight into the drum. Took two or three washes before they felt fluffy again since the old residue had to work itself out, so don't panic if the first couple loads after cutting back still feel off. The cracked door habit the other reply mentioned is a big one too, mine started smelling musty back when I used to shut it every time.

Powder detergent was a game changer for me on this exact issue. My towels had that same waxy feel and it turns out liquid detergent plus my hard water was just coating everything. Swapped to a basic powder and the difference showed up within a few washes. Powders tend to handle minerals better, or at least that's been my experiance. Also check your dryer setup while you're at it. Fabric softener sheets leave a film on towels that feels a lot like detergent residue, and it's easy to blame the washer when the dryer's the one gumming things up. I quit the sheets entirely and just toss in a couple wool dryer balls now. Towels come out fine and they dry faster too.

The thing that finally sorted my towels out was a pack of cheap water hardness test strips from the hardware store. I'd assumed our water was just a little hard, turned out it was way worse than I thought, and once I saw the number I understood why my detergent was basically doing nothing no matter what I tweaked. Worth doing if you're only guessing about your hardness, because it changes the whole answer. Also, on the extra rinse question, I ran it on every towel load for a month while I figured out dosing and honestly the water bump was minor compared to what the old top loader used on a normal cycle.

Update: so I tried the tiny amount trick, like a small splash right in the drum instead of a full cap, and the soapy smell is gone after two washes. Towels are softer but still a bit stiff, so now I'm thinking the hard water angle is real. Grabbed some test strips today. If it comes back bad, is powder really enought on its own, or do you add somthing to soften it?