Watermark doesn't show in exported JPG from LR mobile

Hey there. 

There is something absolutely wrong with creating Watermarks from own images in Lightroom Mobile. 

Even when I go via the export function to render an JPG my watermark doesn't show up. 

It works fine with the "text" or the "Photoshop own" Watermarks option. 

But not with my own logo, which is just stupid. I can select my logo PNG and it shows up in the preview. But after Export it's just not there! BMP, TIF or GIF Formats are just not supported, it seems. Jpg is but has no alpha for transparency, so just useless. 

So I have to hand over the exported picture as JPG to PS-express and there I can add it and then export again WITH watermark. Problem here: It's a lot of unneeded steps to do, first. And secondly PS express just press it in the corner right on the edge and you can not adjust the position like in LR. 

Anyone an idea how I can make this work or is this just a stupid bug, Adobe?

Thanks in advance for any help on this!