1.— Do you notice the problem when viewing at 100% (as opposed to the 24% shown in the screen shot)?
2.— Have you tried the ACR 6.6 Release Candidate?
Also: does setting the brush to "Neu" (New) as opposed to "Hinzufügen" (Add) have any bearing on the issue?
Thank you for the replies, Tai Lao.
1. The behaviour is occuring no matter if viewed in 100%, 24% or any other magnification.
2. No, I haven't tried ACR 6.6. Judging from this thread the problem seems to exist for quite a few versions now.
3. The brush stroke seen on the screenshot was a "new" one. It was the first brush action done to the picture.
I tried reducing the size of the adjustment brush to 2 and place this tiny brush stroke in the outmost corner of the picture. Still the highlights went up. Opening the picture as a Smart Object before using the brush everything is fine in PS. Reopening the Smart Object in ACR and using the size 2 brush far away from the higlights causes the same unwanted beheaviour.
At first I thought it had something to do with having done some adjustment to the exposure before but testing showed that this phenomena happens even when nothing was done the picture prior to using the adjustment brush.