The latest firmware is 1.1.2, you can try update the firmware first.

My apologies, the firmware version that is installed is actually 1.1.3, I've updated the original post. That was my first idea, too, but didn't solve the problem.

Hopefully flying snow can help you get it sorted, 4 minutes seems like a lot. If not, synchronizing time between separate devices like these can be a little tricky depending on the accuracy you need. You might consider using an external automation app to trigger them simultaneously rather than relying on their built in clocks.

That's what I'm going to do eventually... Raspberry Pi to trigger them simultaneously, but until I have that up and running (will take some time), I was hoping to just go with the schedule feature.

It's still very strange. Right now, one out of the three smart plugs is 4 minutes ahead. Yesterday, a different one was 4 minutes behind, but seems to have caught up by now.

Does anyone know how the clock works? Is it depending on the voltage maybe? Fyi, all three smart plugs are connected to the same power strip.

Regarding precision: it doesn't have to be seconds but also, it shouldn't be minutes either.

Anyone else has any idea?

You may try to locate the problem on the devices first. Is it only 1 or 2 of them? Or all 3 of them? If only 1 or 2, I think it's possible that the device you have is faulty or something. While if it is all of them, I would recommend you to contact TP-Link Tech Support by sending them an email so they can look at it.