XML Data in Deep Link to Personal Tab has to be URI Encoded Twice?

Hello ssj_springctin - Thanks for raising your query.
You need to encode both webUrl and data context.

Please refer this code: 

var encodedWebUrl = encodeURIComponent('https://tasklist.example.com/123/456&label=Task 456');
var encodedContext = encodeURIComponent(JSON.stringify({"subEntityId": "task456"}));
var taskItemUrl = 'https://teams.microsoft.com/l/entity/fe4a8eba-2a31-4737-8e33-e5fae6fee194/tasklist123?webUrl=' + encodedWebUrl + '&context=' + encodedContext;

Reference doc: Deep link to an application - Teams | Microsoft Learn

Could you please share your valuable feedback via Microsoft Teams Developer Feedback link.

Please let us know if you have any further query.

Thanks!!