Thanks for the report. The add-on could be much better but I’m afraid I can’t help. You probably know more about this stuff. If you find something useful let me know.
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There’s the issue Evaluate feasibility of using parts of TinyMCE · Issue #2025 · ankitects/anki · GitHub . I’ll wait with updates for this add-on until this issue is closed.
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this add-on is just about bundling tinymce5 mostly with default settings. To warn about unintended behavior such as yours I have a line in the add-on description that the field content sometimes is change automatically. After your post I made it bold.
You could have a look at the tinymce config options at Documentation | Docs | TinyMCE (maybe Table plugin | Docs | TinyMCE) and then adjust the top of the file “template_tiny5_body.html”