Version: 1.43.2 (system setup)
Commit: 0ba0ca5
Date: 2020-03-24T07:38:38.248Z
Electron: 7.1.11
Chrome: 78.0.3904.130
Node.js: 12.8.1
V8: 7.8.279.23-electron.0
OS: Windows_NT x64 10.0.18363

Steps to Reproduce:

1. call in extension's activate function

vscode.commands.executeCommand('markdown.api.render', '[page=42](doc/myfile.pdf#page=42)').then(result => {
    console.log(`rendered markdown: ${result}`);
});

gives ...

<p data-line="0" class="code-line"><a href="doc/myfile.pdf#page42" data-href="doc/myfile.pdf#page42">page=42</a></p>

'=' character was dropped !

2. call in extension's activate function

vscode.commands.executeCommand('markdown.api.render', '[page=42](file://///server/path/doc/myfile.pdf#page=42)').then(result => {
    console.log(`rendered markdown: ${result}`);
});

gives ...

<p data-line="0" class="code-line"><a href="file://///server/path/doc/myfile.pdf#page=42" data-href="file://///server/path/doc/myfile.pdf#page=42">page=42</a></p>

'=' character was not dropped !

Does this issue occur when all extensions are disabled?: Hmm ... extension is needed in this case.