Android 15 QPR2 introduced a bug where the Microphone indicator will sometimes remain active after Microphone usage ends. We've confirmed this issue is present in the stock Pixel OS for both Android 15 QPR2 and Android 16 Beta 3. We're working resolving the regression but haven't figured it out yet.
Here are several upstream issue reports:
https://issuetracker.google.com/issues/388151378
https://issuetracker.google.com/issues/392596949
https://issuetracker.google.com/issues/401832184
It does not mean that apps are actually continuing to use the Microphone. They introduced a bug where the OS can miss that it stopped.