Copy & paste problem - requires multiple attempts

Since you have already tried restarting pboard, disabling Handoff, and clearing the clipboard, first make sure macOS 27 is fully updated in System Settings > General > Software Update. 

macOS Golden Gate 27 was released on September 14, 2026, so it is worth installing any subsequent update that is offered. (Apple Support)

Next, test the problem in more than one application. 

For example, select text in Safari or another application, choose Edit > Copy, and paste it into TextEdit. 

Then repeat the test using Command-C. 

Apple documents both methods as standard ways to copy selected content. (Apple Support) 

If the problem occurs only in one application, that application should be investigated separately.

If Edit > Copy works but Command-C does not, check System Settings > Keyboard > Keyboard Shortcuts > App Shortcuts and make sure Command-C has not been assigned another function. 

If you have a third-party keyboard remapper or keyboard utility installed, temporarily disable it and test again. 

This includes software such as Karabiner-Elements, BetterTouchTool, Keyboard Maestro, or software supplied with a third-party keyboard.

If the problem occurs across applications, test the Mac in Safe Mode. 

On your M3 MacBook Air, shut down, press and hold the power button until the startup options appear, select your startup disk, hold Shift, and choose Continue in Safe Mode. Then test Command-C several times. (Apple: Safe Mode)

If the problem does not occur in Safe Mode, restart normally and review System Settings > General > Login Items & Extensions, particularly third-party items that load automatically or are allowed to run in the background. 

Apple provides these settings for managing background activity. (Apple Support)

If the problem also occurs in Safe Mode, test Command-C in a temporary additional macOS user account. That comparison can help determine whether the behavior is limited to your existing user account.

I would not reinstall macOS at this point. 

These tests provide a more controlled way to narrow down whether the problem is application-specific, related to the Command-C shortcut or third-party keyboard software, associated with startup software, or present in another user account.

One other detail is worth clarifying: your question says MacBook Air M3, while the parenthetical says iMac 24″. The Safe Mode instructions above are specifically for the M3 MacBook Air. 

If the affected Mac is actually the iMac, the troubleshooting sequence is essentially the same, but the keyboard/peripheral test becomes more relevant—particularly if the iMac uses an external keyboard.

Based on your testing, the problem so far seems to occur mainly when using Command-C/V between TextEdit documents. 

You have not reproduced it in Pages or LibreOffice, while Edit > Copy in TextEdit appears to work normally. 

Since you have already tried restarting pboard, disabling Handoff, and clearing the clipboard, I would not repeat those steps.

First, isolate the keyboard shortcuts from the copy-and-paste operation. In TextEdit, try Edit > Copy followed by Command-V, and then Command-C followed by Edit > Paste. 

If one combination consistently works while the other does not, that provides more specific information about where the problem occurs.

Then quit TextEdit completely, reopen it, create two new documents, and use Format > Make Plain Text before repeatedly testing Command-C/V between them. 

This can help establish whether the behaviour also occurs with newly created TextEdit documents. Apple: Create, open, and convert documents in TextEdit

If the problem continues, test the same TextEdit-to-TextEdit operation in a new temporary macOS user account. 

Comparing the results between the two accounts may provide useful information about whether the behaviour is specific to your usual account.

For now, using Pages for simple text documents while waiting for macOS 27.1 is reasonable. I would not reinstall macOS for this problem unless further troubleshooting provides a reason to do so.

Apple: How to copy and paste on Mac

Does this occur for a different user, e.g., Guest ? 

Change Users & Groups settings on Mac - Apple Support

Does this occur after starting up in Safe Mode? 

Sounds good.

I also have this issue macOS 27.0, M2 MacBook Air