The library for the Nextion Number object only accepts uint32_t, but the number object is actually expecting a SIGNED int32_t.
You could certainly modify this in your local copy of the library.
It comes from a time when the Nextion only supported unsigned 32 bit numbers.
I do not have any idea of the timeline for when the online library will be corrected.
The ITEADLIB is being worked on as an ongoing process and provided as some semblance of an example.
Although the ITEADLIB was released, official support is limited to Level 2 to ensure the Nextion hardware works.
Coding questions and techniques should be posted in the Free Chat section where other users can try to assist with any Arduino related items (of which, you will see we try to assist where we can as well)
As this is not a Nextion Editor bug or a Nextion device bug, this topic will be marked as closed.