Thanks for the video and reproduction project. I’ve tracked this down and fixed it for the next release: https://feedback.objo.dev/bug/861
The number wasn’t actually being deleted. On macOS, pressing Tab moved focus to the next NumericTextField and selected its contents, but the same Tab was then incorrectly received as text input. This replaced the displayed number with an invisible tab character, making the field appear empty. The underlying numeric value remained unchanged, which is why the number reappeared when focus moved away.
NumericTextField now ignores tab characters received as text input while retaining normal Tab navigation between controls.