Wanted to add my findings to this topic but it’s closed
I’m using v2021.3.
I noticed when I start play mode with the child objects canvas component deactivated, the anchors of the rect transform I have saved (stretch with parent) are not used and are overwritten with a lower left anchor.
When the canvas component is set to active when play mode is started it is able to keep its settings and scale with the parent canvas whether active or inactive.
The issue in the original thread is ancient and has long since been fixed. If you see similar symptoms, it’s likely a different or new issue.
2021.3 is now only supported with an Industry license. If you don’t have one, it’s unlikely for this issue to get fixed and you’d have to upgrade.
The best approach is to try to reproduce the issue in an empty project. If you can, use the project to report an issue with Unity (assuming you’re on a supported version). Without getting a project to easily reproduced the issue through their bug reporter, it’s unlikely for Unity to find and fix this issue on their own.
If you can’t reproduce the issue, then it’s something specific to your setup and/or your scripts. Try to slowly match the setup in your reproduction project until you can isolate the cause. Then either report the issue or work around it in your setup.
I don’t have this issue with recent versions of Unity. So it’s either something that has been fixed or only happens under specific circumstances.