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.
I am following the guideline at https://docs.nodebb.org/en/latest/installing/cloud/heroku.html
But I have an error after finishing all the steps, please help
Can you run it in a one-off dyno to see what the error is? Perhaps inspect the log? The heroku log doesn't show much for debuging