Your testing establishes an important fact: the same Magic Mouse 2 works normally with an iPad running iPadOS 26, but does not move the pointer or click when connected to devices running macOS 27 and iPadOS 27.
The mouse is recognised and connected on the version 27 devices, while the Magic Keyboard continues to work.
Apple's documentation confirms that Magic Mouse is supported with iPadOS and provides instructions for pairing and reconnecting it. (Apple Support)
Apple also provides standard troubleshooting steps for Magic Mouse connection problems on Mac. (Apple Support)
Because the same mouse works on your iPad running iPadOS 26, your test demonstrates that the mouse is capable of functioning normally.
It also demonstrates that the problem occurs with this mouse when it is used with your devices running macOS 27 and iPadOS 27.
However, these tests do not by themselves prove that macOS 27 or iPadOS 27 contains a confirmed Magic Mouse bug.
I could not find an Apple document confirming this specific problem. Apple released macOS 27 and iPadOS 27 on September 14, 2026, so these are newly released software versions. (Apple)
Therefore, the most technically accurate conclusion is that you have identified a reproducible difference between the operating-system versions: the Magic Mouse 2 works with iPadOS 26 but, in your testing, does not function correctly with macOS 27 or iPadOS 27.
A software issue in the newer operating systems is one possible explanation, but it has not been confirmed by Apple.
At this point, there is no evidence from your testing that the mouse should be replaced as defective.
Keeping the mouse and waiting for a subsequent Apple software update is reasonable, while recognising that Apple has not yet confirmed that a future update will resolve this particular problem.
If the problem continues, submitting a report to Apple with these exact cross-version test results would be useful because it clearly documents the difference: the same Magic Mouse 2, working on iPadOS 26, but not functioning correctly on macOS 27 and iPadOS 27.
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