Technical details:
godot/editor/editor_spin_slider.h declares grabbing_spinner_mouse_pos. Its default value is Vector2(0, 0).
When user grabs a numeric field (not a spinner) and drags, the mouse pointer is made invisible.
When the user stops dragging, the mouse pointer is made visible again at position grabbing_spinner_mouse_pos. But value of grabbing_spinner_mouse_pos is Vector2(0, 0). So mouse pointer re-appears at top left corner.

If we set grabbing_spinner_mouse_pos = get_global_mouse_position() before making the mouse pointer invisible, the mouse pointer reappears at the same point from where it vanished

Difference between behaviour of spin sliders and other numeric fields:
In spin sliders, the mouse pointer appears at the centre-bottom of the Rect. But for other numeric fields, the mouse pointer reappears from where it disappeared.

Imo, reappearing at original position is good as a default. Customisation for specific fields can be done thereafter.
Eg: grabbing_spinner_mouse_pos = get_global_position() + grabber_rect.position + grabber_rect.size * 0.5 is the customization for spin sliders. Removing this line of code, will make Spin Sliders follow the new default behaviour too.

Thanks for your first time contribution! Some notes below

I will make the necessary changes.

Thanks! And congrats for your first merged Godot contribution 🎉