Mermaid diagrams don't always render

### Description of the issue

A clear and concise description of the issue.

### Your environment

* Version of Huly 0.6.388
* Browser (and version) Desktop version
* Your operating system (and version) MacOS Sequoia

### Steps to reproduce

1. Add 'Diagram' block to document.
2. Enter valid Mermaid syntax into Mermaid block.

### Expected behaviour

Always see rendered Mermaid diagram

### Actual behaviour

Sometimes only see Mermaid syntax

Start by reproducing the issue with a Diagram block in a document using valid Mermaid syntax on the reported Huly version and environment. Trace the Mermaid block's rendering path from the document UI; done means the diagram renders consistently instead of sometimes showing only the syntax.

Written by the indexing model from the issue text.