Limitation of large .md failing to save in admin (unless in expert mode)

This came from on some one on the discord channel, who had a large default.md page,

If he copies the file to his pages folder on the webserver, then it displays, if he then goes in to admin , and tries to edit in normal settings, it will said invalid frontmatter in content: and not save the file , but if you then go in to expert mode, I was able to copy the whole document and paste in the same default.md , it will save with no error.

unsure why, that is the solution, maybe a limit in normal mode processing ?
Below is the error recorded in normal mode

Thanks

Reproduce the failure with a large default.md copied into the pages folder, then compare saving it in the admin's normal and expert modes. Start with the invalid-frontmatter error shown in normal mode; done means the same large document saves successfully without requiring expert mode.

Written by the indexing model from the issue text.