How to reproduce:
1. Open the attached project named "URPOverlayCamera" and open the SampleScene
2. Enter Playmode
3. Observe Render Texture RT in Assets folder does not contain text from canvas rendered into it
4. Exit Playmode

Expected result: Renter Texture has the Canvas and the Text rendered into it
Actual result: Render Texture does not contain text from canvas rendered into it

Reproducible with: 7.5.1(2019.4.14f1), 8.2.0(2020.1.12f1), 10.1.0(2020.2.0b10), 11.0.0(2021.1.0a3)

Workaround:
1. Remove the target texture from the Base Camera
2. Enable HookOnOverlayCamera script on OverlayCamera
3. Enter play mode
4. Inspect RT which now contains Both cameras plus the canvas

Notes:
1. Reproducible with URP and HDRP. Tested with HDRP 8.2.0