Since the problem occurs with two different Bluetooth mice, including a new Magic Mouse, I would be inclined to investigate macOS or a software-related issue before suspecting defective mouse hardware. The fact that the problem develops after the iMac has been running for a while is particularly interesting. It could indicate a background application interfering with mouse events, a macOS process becoming unresponsive, or a Bluetooth-related issue.
I would suggest the following troubleshooting steps:
- Test with a wired USB mouse. When the problem occurs, see whether a wired mouse continues to register left-clicks normally. This would help distinguish a Bluetooth-specific problem from a more general macOS input issue. Connecting a Magic Mouse to its charging cable isn't equivalent to testing a wired mouse.
- Test in Safe Mode. Since your 2019 iMac is Intel-based, restart it while holding the Shift key until the login screen appears. Safe Mode prevents certain third-party software from loading and performs additional system checks. Use the Mac long enough to determine whether the problem returns.
- Check background utilities. If the mouse works correctly in Safe Mode, investigate any third-party mouse utilities, window managers, accessibility tools, or other applications that might intercept mouse clicks. Also review System Settings > General > Login Items & Extensions.
- Test with another macOS user account. If necessary, create a temporary user account and test there. This can help determine whether the problem is associated with your existing user configuration or affects the entire system.
I wouldn't prioritize an SMC reset at this stage. Although it is relatively straightforward on an Intel iMac, the symptoms don't particularly suggest an SMC-related problem.
