What version of Hugo are you using (hugo version)?

v0.68.3 on macOS via HomeBrew

Does this issue reproduce with the latest release?

Yes.

Whenever I enable the Table Of Contents in a situation without any endLevel specified in the site configuration:

Extra-Curriculars

Heading Four Here without Heading Three Before It

Technology

Heading Four Here without Heading Three Before It

The Table of Contents comes out looking like this:

I believe the expected behaviour would be for no extra bullet point to appear (or to appear with the H4).

If I add an H3:

Extra-Curriculars

Testing

Heading Four Here

Technology

Heading Four Here without Heading Three Before It

then:

Work-Around

If I manually set an endLevel, the problem disappears. It is only when one is not set that the problems occur.