cannot combine output from 2 cameras, unless viewport x or y coordinate is slightly different

Unity Engine
,
1

Unity 2020.2

Build in render
See screenshots

Setup for camera1
Setup for camera2
Updated setup for camera2 with correct rendering - changed viewport x from 0 to 0.0001

If this is not bug, please explain what i am doing wrong, i did similar setup in before and it was working without viewport difference magic.

6718906--772774--no_bug.jpg
6718906--772774--no_bug.jpg1241×1014 171 KB

6718906--772777--bug_ui_camera.png
6718906--772777--bug_ui_camera.png1222×971 114 KB

6718906--772780--bug_ui_camera_1.png
6718906--772780--bug_ui_camera_1.png1202×938 63.5 KB

2

Update:
After some random creation/deleting of cameras and canvases, problem suddenly resolved itself, and everything start working correctly with the same setup without need to change viewport value. Dont know why this happened in the 1st place.