This is a weird but quite consistent and repeatable bug. Many websites provide a location where you can drag & drop files into to upload them. Dolphin work perfectly as long as the Dolphin window is on the _same_ monitor as Chrome (I haven't tested with other browsers). If Dolphin window is on another monitor, the drop fails and I just get a white page in Chrome. This caused me panic a few times as I thought I lost my data (e.g. when replying to forum posts).
Did you try with another application dragging to Chrome, e.g. Nautilus or any other non-Qt app? Did you try with another browser? Just want to rule out it's a Chrome bug.
I just tried with Nautilus and it works perfectly & repeatedly. However, I also tried again with Dolphin and now it works, but occasionally I get this problem.. so not sure what's going on now... At one time, Chrome displayed the image in the tab that I dragged the file into.
This bug is still valid. It's quite inconsistent, but I'm getting a failed drop about 50% of the time. In order to fix it, I have to drag the files back into dolphin, and then back out again until it works. Upon further testing, it appears dolphin is advertising the drop as a string with a "file://" url, which websites in chrome can't do anything with due to security restrictions in most browsers. I believe this has something to do with the copy/move menu that pops up when dragging between dolphin windows, as dolphin has some weird drag-n-drop behaviors to make this work.
This problem is still there. I'm using Chromium 100 and have 2 Monitors 1 of which is disabled. Starting KDE with X the menu pops up just fine and I can click on copy and it will save the image I dragged on Dolphin from Chromium. Starting KDE with Wayland the menu doesn't pop up and no file is created but I believe that it makes my laptop's fans spin up due to doing something. This has to be fixed. It's an essential thing for webbrowsing and I can't set my scroll speed in X, so I have to use Wayland. Make it work. It's 2022. You had plenty of time.
I can confirm the bug is still present and quite annoying.