I have updated drivers, BIOS, and did a factory reset on the monitor. One of these did the trick - I'm back to 60 Hz again. Thanks to DELL Support staff for walking me through this.
I have an Latitude 5520 (up-to-date drivers and BIOS) directly connected to a Dell U3219Q 32" 4K Monitor (latest firmwareM3B110) over USB-C (using either of the Thunderbolt ports on the laptop). I'm using the Dell-suppplied cable that came with the monitor. I'm getting only 30 Hz frame rate on the monitor. Previously, I'd been using a Latitude 5510 (USB-C only, no Thunderbolt) with the same monitor (and cable). In that configuration, I got 4k at 60 Hz. Any pointers what I could check or change to get back to 60 Hz on the monitor?
Latitude 5520 and U3219Q Monitor just 30 Hz (no Dock)