Table of contents bug - num list headings are on multiple lines in the TOC

I'm working on some documents with multiple headings and table of contents. As part of the formatting, most of the headings are numbered using the num list format, like so:

However, a strange issue has been occurring where, when I set up the table of contents, some of these headings and subheadings spill over multiple lines in the table. I can't figure out why this might be—every heading has the same formatting as each other, and every subheading has the same formatting as each other, yet only some of the headings and subheadings experience this bug in the table of contents. Most are normal. See below for a simulated example of what I mean:

As can be seen, the actual titles are on separate lines from the page numbers, and some of the numbers are on separate lines from their titles. The ideal format should be, say: "1.2 [Title] [leading dots] [page number]", all on the one line. I've had a look at the headings to see if there are any formatting or paragraph inconsistencies, but I can't find any. I'd like to get some help with figuring out what might be causing this issue.

I apologise, I know this is a bit vague, but I really don't know how to explain it any other way. I'll appreciate any help you can offer, thank you.