Macbook Pro sometimes uses incorrect 3440×1440 resolution for 5120×2160 external monitor (TB3)

I have a 15" 2019 MacBook Pro and a LG 34BK95U monitor (business version of 34WK95U – same hardware, longer warranty).

The correct resolution of the monitor is 5120×2160 and sometimes everything works at that resolution. But, sometimes when I connect my MacBook (via ThunberBolt 3) or when it wakes from sleep already connected, it sends a 3440×1440 image to the monitor, which looks fuzzy and gets a warning from the monitor about the incorrect resolution.

When this happens, the MacBook doesn't even give me the option of using the correct resolution in system preferences – even if I Option-click on Scaled, the highest resolution available is 3840×2160. This makes me suspect it's using DisplayPort 1.2 instead of 1.4, because I believe that's the maximum resolution DP 1.2 supports.

Sometimes disconnecting and reconnecting the cable fixes it, sometimes switching to a different TB3 port in the MacBook fixes it, sometimes rebooting the MacBook or the monitor fixes it, but nothing reliably fixes it, and sometimes I go through all of the above and still have the wrong resolution. (To reboot the monitor, I just unplug it from the wall, wait a few seconds, then plug it back in.)

I mainly use the ThunderBolt 3 cable that came with the monitor, but I also tried a short passive TB3 cable – same results.

It also forgets the arrangement of the display relative to my MacBook's screen when using the wrong resolution; it remembers when it uses the correct resolution.

In ? > About This Mac > System Report… > Graphics/Displays, it has the display connected to the Radeon Pro 550X with the following info:

LG HDR 5K:
  Resolution:   3440 x 1440 (UWQHD - Ultra-Wide Quad HD)
  UI Looks like:    3440 x 1440 @ 60 Hz
  Framebuffer Depth:    30-Bit Color (ARGB2101010)
  Mirror:   Off
  Online:   Yes
  Rotation: Supported
  Automatically Adjust Brightness:  No
  Connection Type:  Thunderbolt/DisplayPort

I'm currently on macOS Catalina 10.15.4, but this also happened on previous versions, including Mojave releases.

It also sometimes turns on HDR mode when using the wrong resolution, which just has the effect of making everything a little darker. I think that started with the Catalina update.

After reading Incorrect external monitor resolution detected, true resolution not available I went into the Color Sync Utility and Verify found one issue, but Repair couldn't fix it:

Searching for profiles...
Checking 35 profiles...
/System/Library/ColorSync/Profiles/ITU-2020.icc
   Header message digest (MD5) is not correct. 
   The file is locked. Could not be fixed.
Repair done. 0 out of 1 profiles fixed.

I also noticed that under Profiles > Computer > Displays there are 4 different "LG HDR 5k" entries. Not sure if that's related or not.

After reading External monitor has wrong resolution I tried resetting the NVRAM / PRAM, but that didn't fix it either.

I also have a Windows PC connected via DisplayPort and it always works at the correct resolution.

Update: It seems to be getting worse. I've only been able to get the correct resolution once since posting this. And it went away as soon as the computer went to sleep.

I tried connecting a CalDigit USB-C Pro Dock (which is actually Thunderbolt 3, despite the name) and then connecting it's DisplayPort output to the monitor. The port on the dock is labeled 4k, but it can actually output the full 5120×2160 resolution – but only at 30hz. The low refresh rate is annoying but better than the fuzzyness of 3440×1440@60hz, so I'm going with it for now. There were a couple of times where I had to disconnect and reconnect the DisplayPort cable to get any image out, but one reconnection fixed it each time, making it far more reliable than the direct TB3 connection.

I've also ordered a generic USB-C to DisplayPort cable, so I'll update again after I've had a chance to try that.

Update 2: USB Type-C to DisplayPort cable behaves about the same as the dock – correct resolution, but only 30hz refresh rate.

Update 3: I got a hold of a Windows laptop with thunderbolt 3, and it worked perfectly on the first try, and again after I disconnected it and reconnected it. So I think that points to a problem with my MacBook.