After searching the general forums and in area51 I thought this the best place to continue this discussion. The thread here
also is good. I've done very careful debugging and there is indeed a genuine problem with plupload and current browsers. I'm happy to present my debugging for your review, but I thought I would just cut to the chase with my results and see if you agree or disagree as some of you may already know about this.
I'm not a developer but I know a lot of s/w. I have a thick skin so if you think I'm wrong let me know. Also, I believe this explains some of the confusion posted previously, why something worked for someone and not others, etc.
The bottom line is that current browsers now support auto image rotation natively. Plupload always draws to the browser's canvas which will, in turn, auto-rotate the image. However, plupload is designed to also rotate the image as part of its resizing code, independent of what the browser may or may not do. You'll essentially get a double rotation affect. The displayed image will not be correct, certainly not what you expected. Note that an original image that hasn't been rotated (orientation bit = 1) will not be affected by any of this.
Chrome 81, for example, was released about beginning of 2020 with first-time support for auto image rotation. If you were to use Chrome before this, then the combination of phpBB + plupload would work fine. Likewise, newer releases of Edge does auto-rotate and so on.
Drag-and-drop photo uploading, resizing, and EXIF stripping are incredibly important to me on my forums. To test my conclusions I wrote a patch that seems to work. I hesitate to post my patch as it modifies a core file and plupload (the code mods are very small though). I'd like to first hear from y'all since you know a lot more about how all these pieces fit together than I do.
I upgraded my forums from 3.2.0 to 3.3.2 and I was hoping the rotation problem would somehow fix itself. When it didn't I started this debugging.
My other concern, if you agree with me, is that going forward there's another possible issue. plupload has been commercialized and is no longer supported in the public domain. I looked at this company's licensing and I'm very suspicious about their lack of support and that it'll even work with the new browsers. At any rate, not sure what phpBB's position is on this. Again, I'm not expert on all this but surveying github and the websites seems to indicate this is what's happening.