What version of Hugo are you using (hugo version)?

$ hugo version
0.95

Does this issue reproduce with the latest release?

I am using the docker version, so waiting for klakegg/docker-hugo#64 to be merged

Problem

Based on HugoBlox/hugo-blox-builder#2708, we observed

<span> instances are being inserted by Hugo which we would assume shouldn't happen when Hugo is configured with noHl: true. It's either a Hugo bug with the noHl option or there's an opportunity for Hugo team to allow the Hugo code block parser to be used to detect Mermaid diagram code blocks without highlighting any code or injecting any <span>.

The <span> instances being inserted cause rendering issues for code blocks in markdown slides.