There are a few reasons that can cause this problem.
Not allowed HTML markup
WordPress can remove extra formatting when you switch between visual and text tabs. This case can be simply tested in the default WordPress editor as shown in the example below:
Your text can look different on the front end if that is the case. A list of allowed tags can be found here.
Unnecessary HTML markup
Some of the 3rd party resources or text editors such as Microsoft Word can have HTML tags in the text, these tags are not visible however when you copy text and paste to the WordPress editor all tags are also copied. You can see these tags in the Text tab of the WordPress editor.
To prevent it you can use online tools such as striphtml.com to remove all HTML tags from the text.
Broken HTML markup
Text on the front end may look different if there is a broken HTML tag in the text, it also may affect all other content on the page. Here is an example of the correct HTML markup:
<h1><a href="#">Link Text</a></h1>
If one tag is missing it might affect the whole content on the page. Here is an example:
<h1><a href="#">Link Text</a></h1
You can use an online HTML validator, for example, validator.w3.org to check if your code is valid and fix the errors.
Issues related to CSS code
If you see that font size, color, alignment or similar properties do not work on the front end this might be related to an incorrect CSS code. In this case, you will need to make sure that the CSS code is valid and doesn't have any mistakes.
To check CSS code please follow this article.