We have just had this wallpaper fitted by a professional. It’s on new plaster, which was sealed with a wash coat of emulsion and allowed plenty of time to dry. The staining has only appeared down one and a half seams but is really obvious. There is no wallpaper paste on the top of the paper as it was really well washed down, and the texture of the wallpaper when you rub your hand over is smooth - not sure what’s up! Has anyone experienced anything like this before? (The stains are on the right hand seam and half way up the left hand one!)
Since you stated in your comment that it looked fine at first but developed these stains after drying I think there is some chemical reaction that occurred in those spots.
Plaster can have hot spots that need to be neutralized. I had a job where every area that was fixed with plaster was hot. After installing commercial vinyl a reaction between the adhesive and plaster began. In 24-48 hours the hot spots were blistering the vinyl off the wall.
If you are only seeing staining at 2 seams it may be that those seams were not adhering and the installer opened them back up and brushed in some premixed vinyl adhesive that caused staining. Some wallpapers can not tolerate vinyl adhesive and a bleed through will occur. Since this is a mural I would suspect it is made of thin delicate paper and you should carefully read the instruction sheet to be sure all preparation steps were taken and if it cautions against using certain types of adhesive.
It looks to me like the problem was caused during installation. You could guess that this is some other source of moisture, but it wouldn't travel that far from the seam, and if it's coming through the paper then it wouldn't tend to happen at the seams. I'd get the pro back, point out the problem, and see what they say.