If in the text editor I select a part of the text, a sentence, to set it for instance to Heading 1 all the text below it is also set to the style.This is not correct.
The only way to correct is is to go to text mode an manaully edit the html which is not the way to go for people who do not understand html
What to do to fix it?
Thank you for your question.
This is not the theme issue but the WordPress issue. You can confirm it yourself by using any default WordPress theme like Twenty Seventeen.
The issue occurs when the two lines are too close to each other and if you add a line of space between two lines then the issue doesn’t occur.
The issue may be resolved in the future WordPress update.