I have the Android version, but I had the same problem.
What worked for me: go to a WhatsApp conversation and start typing. As soon as the keyboard appears, it has a little cogwheel at the top, in the row of microphone, colour palette, etc (on my phone above the letter ‘u’).
Tap that, and it will give you all kinds of options, including actually switching off autocorrect.
I hope this works on e/OS too 