Often a sign of a growing clog. The door doesn't unlock until the water sensor says there is no water. If the last bit of water is draining slowly, it can delay unlock.

Three places to check a front loader:

1. The rubber gasket for the door has small holes in the folds that get clogged.

2. The lint/large item trap is getting full. Most washers have a little door you can open and pull that out for cleaning.

3. Worst is there's a ball in the drain hose at the bottom of the tub where the tub connects to the drain pump. It works as a check valve. It can get clogged. You need to get access to the motor area of the washer. Often massaging that ball in the hose is enough to let it drain. I'm fully aware of what this sounds like.

I'm more confused by how my machine can say 40mins left but then 40mins later it's still got 30mins and is actively washing.

The older I get the more I appreciate usability over a cheap price tag.

Our current washing machine demands the gathering of an audience by loudly singing the song of its people a full minute or more before it will let you open the damn door. It also has no respect for human arms, requiring a traversal from far left (soap dispenser ), to far right (power on), to centre (program select), then back to the far right for temperature and spin adjust and finally cycle start. Put the damn controls in a logical order across the top of the machine for fuck's sake!

Fellow lemmings, I must know if your machines also display egregious "remaining time" values? Like, mine will say 2 minutes, then later go "ahhh it will be at least 13 more bro" as if gaslighting us. Is this a general issue with the apparently complex science of cleaning textiles?

It may be the wax motor cooling down: https://en.wikipedia.org/wiki/Wax_motor#Laundry_washing_machines

You can open top loaders while they're running to add something you forgot. They're simpler and so more reliable machines, also easier to repair.

But, if your cat likes the front loader, then you're stuck with that.

I definitely read that a women was keeping the door locked to the laundry room until the machine stopped... The internet has ruined me

The machine beeps when the software cycle completes but often the mechanical process termination lags behind this due most commonly to sunspots. Happy now.

Aside from making sure check everything else mentioned in the comments, try pushing the door in before opening

That's funny. Our washer sings as little song when it's done but I can honestly say in many years of ownership, I've never gotten up immediately to empty it. I wonder how long mine takes to unlock.