I suggest taking a look at your html sanitization settings make sure html isn't allowed in markdown settings.
Next step would be to move the other 2 topics from that category and check if you still see broken markup.
When we used to use IPB, we had a handy feature where general public members could enter a forum and only see the topics they have created themselves (used to be handy with ban appeals and reporting people without judgement)
Is there a way we can do this with NodeBB?
Not at the moment without a custom plugin.