This has to do with the multilevel numbering used for your headings that get included in the TOC, I suspect. If the numbering is not from your headings but is only in the TOC, write back. The following will apply, regardless.



Multilevel numbering in Word is easy to mess up. It always has been. Take a deep breath.


Automatic Paragraph Numbering all starts with the Define New List Style Dialog.
You use that to create and name a list style, within that dialog, when you format numbering you go to the Define New MultiLevel List Dialog. The key is to assign a separate existing paragraph style to each level of numbering. The styles can be built-in styles- or custom styles.

If you use automatic paragraph numbering or bullets read Shauna Kelly's directions on numbering and bullets. Start with How to create numbered headings or outline numbering and her parallel page How to control bullets. For large documents you must follow these directions or you will lose your hair!
(Mac version: Outline Numbering in Word for macOS - Best Practices - Brandwares).
For styles attached this way, you also control the left indents through the Define New MultiLevel List dialog not the Ruler or the Modify Style dialog.

You first want the paragraph styles existing in your document.
These can be built-in styles or custom styles or a mix of the two. You can, if you want, modify their formatting later. Then you go to the Define New List Style (Not Define New MultiLevel List! You will get to that dialog in the process, though.) Shauna Kelly's page uses the built-in heading styles. This can be convenient but the process works with any existing paragraph styles including your custom styles.

This may seem a bit convoluted at first, but it really is not. Just follow the steps. Shauna Kelly's instructions use the built-in heading styles, but you can use any existing paragraph styles including your custom styles. There are, however advantages to using the built-in heading styles when you create a Table of Contents. Here are some more advantages: Why Use Word's Built-In Heading Styles? by Shauna Kelly Note, you can modify these built-in styles to look exactly the way you want.

You want to do this even if what you want is a single-level list if you want the most control over your list.

Videos on this

https://youtu.be/niD6VXPvAyU?t=487
https://www.youtube.com/watch?v=EICWOeIhsR4

The basic idea is that the numbering is set using the Define MultiLevel List dialog with each numbering level being attached to an existing paragraph style. Once you have this set up, you should not use the buttons for numbering in the Ribbon but rather apply the appropriate style for that level.

You can save a document with this as a template for future documents if you want so you will not need to do this every time.


===================
If the numbering is not in the document headings but only in the TOC, attach the numbering to your TOC styles.
TOC Tips and Tricks by Suzanne Barnhill, MVP