Restarting my iMac did not fix this issue for long. Today my iPhone 6s disappeared from the iTunes device list. Then my iPad Pro disappeared. I restarted iTunes, and the iPad came back, but when I clicked on its icon, iTunes quit unexpectedly.
All my devices are on the same WiFi and all work fine in all other respects.
Hmmm. Here's a possible clue. I opened AirPort Utility, and see that my AirPort Express (extender) is listed, but my AirPort Extreme (should be the root of the WiFi system) isn't. I'll try a power cycle of the Extreme.
Well, power cycling the AirPort Extreme (I had no access through AirPort Utility to reset it.) brought it back, but lost the AirPort Express. Power cycling the AirPort Express then brought it back and my WiFi network looks right now in AirPort Utility. I'll see what happens with iTunes.
Here's something that looks helpful:
https://9to5mac.com/2016/06/21/airport- ... y-updates/
Turns out I didn't have my AirPort Utility set to check for updates automatically nor to monitor AirPort device status.
[More time passes]
And that didn't work either. AirPort Utility still reports my WiFi network as normal. But just now I checked iTunes and all my iOS devices showed up. I closed the iTunes window and iTunes quit unexpectedly again. Upon restarting it, my iPad Pro was missing. Restarting iTunes again and restarting the iPad Pro did not bring it back, but telling the iPad Pro to forget our home WiFi network, then rejoining it did bring the iPad Pro back to the iTunes device list.