I am having this problem as well on my Apple Watch Ultra 1. It was being very inconsistent, especially if I took the watch out of range of the phone (even though I have a cellular plan).
I will have to report back on if this fixed the issue or not, but at least a temp solution has been to go into the watch app on the phone and switch the complications to something else, and after they've displayed the new complications on the watch, switch them back to what you normally use. This has at least populated them with the current info, though I don't know if it's a permanent fix yet.
Update: This was not a permanent fix. It just populated the current data. A few minutes later, that information was no longer accurate. Pressing the complication opened the app and showed the current info, but even then the complication did not update with the new info. A few minutes later and the complication just went back to showing the dashed lines.