The TOC functionality hasn't changed from 2016 to 2019 so what you are reporting doesn't sound like normal behaviour. I can imagine it might be possible to recreate the issue if your document is set to update styles automatically but this is a very long shot and would need a lot of non-standard things to be in place.

What format was the file saved in when it was transferred to the Office 2019 machine?

You may need to upload a copy of the file so we can see what you have in it.

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Andrew Lockton
Chrysalis Design, Melbourne Australia