This question has already been asked before, but the answer - the textpattern plugin - applied to version 5. I am now using version 6 with no plugins active, but still the editor replaces text between asterisks as italic. How can I disable this behaviour?
The logic in the textpattern plugin was moved into the core editor in TinyMCE 6. This is covered in the Migrating from TinyMCE 5 documentation page.
If you want no text patterns at all you need to set the text_patterns configuration option to false:
text_patterns: false
as documented here https://www.tiny.cloud/docs/tinymce/latest/content-behavior-options/#text_patterns.