I use the built-in headings (Heading 1 etc)  associated with Outline
Numbered. I don't usually have problems.

I created a docuemnt and the first word in each heading was underlined
in red to note a spelling mistake. I know the word is not misspelled
so I right-clicked to look-up. The first word in each heading has a
digit and a number before it.

For example, on screen the first Heading 1 it appears as "Section 1
DEFINITIONS".
In the Search for box, it appears as 0BDEFINITIONS.
The next heading is heading 2 which appears on the screen as "1.1
Unless..."
In the Serach for: box, it appears as "8BUnless"

It prints OK, but it's very annoying that the first word in each
headng style throughout the entire document has this problem.

There are two parts (1) a digit, and (2) a letter. The digit increases
sequentially throughout the document while the letetr remains constant
at B.

Any advice?