iMac keeps forgetting my iPhone

Last week, I posted about a problem where my iMac only partially recognized my iPhone. The details are here https://discussions.apple.com/thread/252649309, but a quick summary is that when I plugged my phone into the iMac, the iMac recognized that the phone was connected, but I couldn't do anything with it. The Check for Update button was grey, the usual menu didn't appear in the finder window when I select the phone in the side bar, and it didn't sync. Photos are in the link above. This was solved by upgrading my phone to iOS 14.4.2.

HOWEVER, now it's happening again. I typically power up the iMac, and while it's booting, I plug in the phone. For the last several days, I've been getting the same partial recognition as before. So far, I've been able to resolve it by rebooting my phone and plugging it in again, but is that how it's supposed to work? How can I get my iMac to always recognize my phone?