Some users in our company have reported a similar issue with the Native Mail app on iOS version 18.1.1, which is not managed.
Microsoft has released an updated version of the Intune SDK (details below). However, this update should not affect the Native Mail app.
"We recently released updated versions of the Intune App SDK and the Intune App Wrapping Tool. Included in these releases (v19.7.5+ for Xcode 15 and v20.2.0+ for Xcode 16) is the support for blocking screen capture, Genmojis and writing tools in response to new AI features in iOS/iPadOS 18.2.
How this will affect your organization:
For apps that have updated to the latest Intune App SDK or Intune App Wrapping Tool versions screen capture will be blocked if you have configured “Send Org data to other apps” to a value other than “All apps”. To allow screen capture for your iOS/iPadOS devices, configure the Managed apps app configuration policy setting “com.microsoft.intune.mam.screencapturecontrol" to Disabled."
I will contact Apple support to investigate further.