Custom Extension for JSON Completion Does Not Work in Double Quotes

I'm developing a VSCode extension that should enhance JSON editing with additional code completion. Therefore I added four dummy suggestions (foo, foo1, foo2, foo3) to start testing. When typing a colon after a JSON key it works as expected:

Also when I trigger completion in an empty object it works as I want it (including suggestions from other extensions):

However when I trigger completion within double quotes (no matter if they contain text or not) my suggestions are missing:

When I set a breakpoint in my CompletionItemProvider execution pauses as expected in both cases. So it is called correctly and it returns my suggestions for sure. I looked in existing JSON editor extensions (even VSCodes default one) to get advice how to implement it correctly and to see if the default JSON extension maybe overwrites all suggestions within double quotes (even though that would break the whole extensibility idea in my opinion) but couldn't find anything.

To reproduce it is enough to scaffold an empty extension as described here.

Here is my extension.ts:

// The module 'vscode' contains the VS Code extensibility API
// Import the module and reference it with the alias vscode in your code below
import * as vscode from 'vscode';

class OpenApiCompletionItemProvider implements vscode.CompletionItemProvider {
    public provideCompletionItems(
        document: vscode.TextDocument, position: vscode.Position, token: vscode.CancellationToken):
        Thenable<vscode.CompletionItem[]> {
        return Promise.resolve([
            new vscode.CompletionItem('foo', vscode.CompletionItemKind.Class),
            new vscode.CompletionItem('foo1', vscode.CompletionItemKind.Property),
            new vscode.CompletionItem('foo2', vscode.CompletionItemKind.Interface),
            new vscode.CompletionItem('foo3', vscode.CompletionItemKind.Class),
        ]);
    }
}

export function activate(ctx: vscode.ExtensionContext): void {
    ctx.subscriptions.push(
        vscode.languages.registerCompletionItemProvider(
            { pattern: '**/*.openapi.json' },
            new OpenApiCompletionItemProvider(),
            ':',
            '"'
        )
    );
}

// this method is called when your extension is deactivated
export function deactivate() { }