Svelte store contains suddenly contains the wrong data and also the correct data

Question

Sorry for the vague question.

Got a store which I filled with data. As far as I can see the data that I put in is correct. I've used many breakpoints.

My site contains 2 +page files, so let's call them page 1 and page 2.

I set the store on page 1 like this inside an async method:

async function register() {

connection.on('ReceiveLlmResponse', (_, fileName, contentType, fileContent, oldFileContent) => {
            let diffDataStructure = CreateDiffDataStructure(oldFileContent, fileContent, { ignoreWhitespace: true });
            let calculated = calculateLineNumbers(diffDataStructure);

            diffStore.update((current) => {
                const diff = {
                    id: current ? current.length : 0,
                    fileName: fileName,
                    diffs: calculated
                };
                console.info('info', current);
                if (current) {
                    console.info('went into place where should not come', [...current, diff])
                    return [...current, diff];
                }
                console.info('retyurned correctly', [diff])
                return [diff];
            });
}

In those logs, the data is correct at every step. Also correct using the debugger. Then it goes to the next page:

It first stops here:

diffStore.subscribe((value) => {
        if (value === null) return;
        diffDataD = value; <<< here
    });

And the data in value is correct. Then it goes to here:

<div class="column-container">
    {#if diffDataD && Array.isArray(diffDataD)}      <<<< breakpoint over here
        {console.log(diffDataD)}
        <div class="code maxxed">
                {#each diffDataD as diffItem}
                    <h2>Old: {diffItem.fileName}</h2>

And for some reason the data inside diffDataD looks nothing like the data that I put into the store.

So here's an example of correct data:

0: diffs: (13) [{...}, {...}, {...}, {...}, {...}, {...}, {...}, {...}, {...}, {...}, {...}, {...}, {...}]
fileName: "admin.controller.ts"
id: 0

And her an exampl of the wrong data which makes my code break (added as .txt file):

diffDataD: [
  null,
  null,
  [{...}],      <<< this array of objects is my correct data
  undefined,
  (diffId, diffItemId, index, old) => {...},
  function handleTextEdit(diffId, diffItemId, index, event) {...},
  function handleTextBlur(diffId, diffItemId, index, event) {...},
  (diff, diffItem, oldIndex) => {...},
  (diff, diffItem, oldIndex) => {...},
  event => handleDelete(event),
  event => handleDelete(event),
  function textarea_input_handler(each_value_6, mergedIndex) {...},
  (diff, diffItem, mergedIndex, event) => {...},
  (diff, diffItem, mergedIndex, event) => {...},
  (diff, diffItem, newIndex) => {...},
  (diff, diffItem, newIndex) => {...}
]

I'm lost. Why does this happen. I don't understand at what point this goes wrong.

I put logs at every step in my code. I used a debugger. I disabled parts of the code.