WordPress auto spacing issue is one of the most annoying issues that happens when you try to edit post or text widget by adding P (paragraph) tags.And ,unfortunately,because of some insignificant glitches in Tiny MCE (integrated WordPress editor) some of formatting tags don’t appear at all that causes text warping etc.
There are several solutions in this case:
1.Use additional Tiny MCE Advanced https://wordpress.org/plugins/tinymce-advanced/
(improved version of original integrated WordPress editor);
2.Avoid switching VISUAL/TEXT mode while editing post or text widget;
3.Add the code below to child theme functions.php,then save changes
// remove <br> tags from text widget content, from 4.8 version WP adds these tags
remove_filter('widget_text_content', 'wpautop');