[2D] Problems with the Animation Controller

Hello guys,

I'm trying to create a very simple idle animation, but it fails completely.
In this example I just would like it to rotate the front foot of my character.

I've done the following steps:
- creating a folder named Animation inside of Assets
- adding an Animation Controller inside of the folder
- clicking on "Create" and name it Idle.anim

- by pressing the button "Add property", I chose the desired texture (front foot) and selected rotate
- the default key frames are on 0:00 and 1:00
- now I click on "Record", jump to 0:40 and simply type in a 5 (Rotation Z) inside of the Inspector
- the rotation of the foot looks correct and I press again "Record" to end the record

Well, it does NOT work.
After I playback the freshly created animation, the rotation Z goes up to over 150.
It looks weird and it's not what I wanted to achieve.
I also went to "Curves" and set everything to "flat", but no change either.

Sure, I can correct everything manually afterwards, but is it really necessary?

Is this some bug or am I doing something wrong?
At least that's how I saw it in tutorials.

Thanks for any help!

Edit
Unity Version: 2018.3.0f2

Greetings