This is something what Athom has to fix, not the App Developer of the Homey Apps.
So please contact Athom Support → Support | Homey
Btw, negative values can be entered in the WebApp.
Edit
This is what @TedTolboom, the App Developer of the Aqara App, wrote ten days ago about the “external sensor” in a other topic:
So it’s on his To-Do list.