Check code block styling. Some styling was being applied which has been radically changed in Docusaurus core - if styling changes are needed override in custom.css.
Check whether your custom redirect plugin is needed - try replacing with @docusaurus/plugin-client-redirects - lower maintenance burden on you to maintain going forward.
Build test passes - to check/confirm:
custom.css
.@docusaurus/plugin-client-redirects
- lower maintenance burden on you to maintain going forward.