Monitor at wrong resolution with Thunderbolt HDMI adapter

Hey folks, I have an HP 27xi monitor, which has a recommended resolution of 1920 x 1080 @ 60 Hz (1080p). I've been using it with my 2015 MBP for a long time, going directly from the HDMI port to the HDMI port on the monitor. It worked well. My 2017 MBP only has Thunderbolt, so I'm using an official Apple Thunderbolt HDMI adapter to make the connection. When the lid of the laptop is open, the image on the monitor is fuzzy and full screen videos stretch slightly past the screen. With the lid closed, the image on the monitor goes a good way past the screen. I don't have "zoom" enabled. I've tried both "Default for display" and "Scaled", including Option-clicking on "scale" to get alternate resolutions. Nothing seems to help. Any ideas are welcome.