Reproduction steps (GIF attached):
1. Open the attached project.
2. Build it as standalone.
3. Click 'Instantiate Canvas 2'.
4. Click '<' near 'sub canvas1' text until 'SubCanvas 1' leaves the screen.
- 'SubCanvas 2' may become invisible now.
5. Move 'SubCanvas 2' around clicking '<' and '>' buttons near 'sub canvas2'
- 'SubCanvas 2' becomes invisible on some spots.

Expected behavior:
'SubCanvas 2' is visible all the time when it is on the screen.

Note:
Higher reproduction rate using full screen.
Sometimes 'SubCanvas 2' gets visible again when moved around.

Reproduced with:
5.5.5f1, 2017.3.0b2
iOS (easiest to reproduce) - 'SubCanvas 2' becomes invisible when 'SubCanvas 1' leaves the screen.
macOS Standalone, Windows Standalone - may require 'SubCanvas 2' movement.