Speaker phone turns off randomly during calls

My iPhone 12 Pro frequently turns off speaker phone while on calls at random moments. I don’t have any Bluetooth accessories connected to phone when this occurs. It started with a software update a year or two ago (maybe iOS 14.5), and I was hoping the issue would resolve itself with future updates but it has not. When this occurs, the speaker switches itself from speaker to handheld speaker output, HOWEVER, the phone app STILL shows the speaker button HIGHLIGHTED as if it is on speaker. To fix this, I have to click the speaker button to set it back to handheld, and then click it again to put it back to speaker. My iPhone is always on the latest available iOS version available but this issue has not been resolved.