Formatting changes when you click enter

Hello!
I have a huge Problem when writing a text.
After saving a formatting that I also set as the default for the entire project, it’s not working properly.

Every time I press Enter for a new paragraph, the formatting changes – my formatting is completely ignored!

Please, can someone help me?

Assuming you’ve only now set your default formatting, whether it be for all or the current project, that default will only apply to new documents going forward. Check out the Document > Convert > Text to Default Formatting option for existing documents, including the one you’re working on, once you’ve set a default format.
And look out for the tick box choices if you have any form of specialised formatting you don’t want to ruin.