What Lightroom version are you using when you see this phenomenon?
The same sort of issue was reported back in February and the workaround was to export from the Library module.
I have The 5.6 version and i export my files from Librabry and change my file size to 800px larg size and add a watermark. sometimes i do 3 export at the same time cause i split my wedding to folders bride- preparation- groom-ceremony.......
if i change the priority of lightroom on the task manager to HIGH can fix that? is it any othe way o exort a folder and export the files with subfolders as my Path tree or simply a bug to pla and fix it.
I am just another user like yourself, and Googled to find the Feedback thread about the issue so I don’t have much other advice what to do. You’ll probably have to figure out any workarounds to the issue, yourself, as I don’t have a large batch of photos to export. If you can find something that makes the problem go away it’s probably worth reporting to Adobe on the feedback site, with the particulars about what you’re doing.
I wonder if there is some issue with using a PNG watermark and if you used a text watermark it wouldn’t happen. Or can you make a PNG watermark that covers the entire 800px photo with transparency over most of the area, and see if that makes it worse or better. Are you creating the PNG with Photoshop? Maybe use different compression options or no compression for the PNG and see if things work better. There is obviously a bug somewhere but without Adobe being able to reproduce it, it will be hard to track it down.
I will try to play with my watermark file type. now is on PNG format.Also i will try the other way that you said to me. Thank you for time i will publisg again my experience to my issue.