That's expected behavior for outputting HTML on Teams and so I'm sure it's the same on Slack.
As far as the character not being displayed, that's gotta be an issue on the back-end outside our control with whatever ServiceNow does to render the HTML before turning it into an image. Could be worth opening a case to ServiceNow about that...
I would first recommend though you either to put your output text in the normal "Response message" box where you can do basic formatting (bold/italics/link/list) - you can use script variables in here to be somewhat dynamic - or otherwise if you absolutely need to script the output, try Markdown, which should render properly as formatted text (not an image) in Teams/Slack if I recall.
