Jetbrains have responded here with the following:
This is likely
RIDER-116113, a known regression we are planning to fix soon.
Additionally, please be aware that
- Rider has a mode enabled by default that syncs the tool window states between opened solutions (unique among IntelliJ products)
- This mode is known to be buggy
- We are planning to rework it so it doesn't sync the tool windows in runtime, but only on solution opening (thus liquidating most of the known problems)
- But what you see is most likely a bug we'll fix soon. For now, you may offer the users a workaround of disabling the flag Window → Layout Settings → Use the Same Layout for All Projects.