I have the latest Creative Cloud versions of Lightroom and Lightroom Mobile on my Ipad. On my desktop I have created a collection which I have put in a custom order. On my Ipad I go into that collection and set the order to custom order, which I presume means the order I have on my desktop. However, the photos on the Ipad seem to be just random photos from the collection THEN the rest of the collection is in the correct order after that.
Do you have virtual copies in your collection?
Could you please send send me a LR Desktop +Mobile diagnostig log - best as a private message with a downloadable dropbox link
You can trigger the Lr Desktop diagnostig log via LR Desktop preferences -> Lightroom Mobile and when you hold down the alt key you will notice a generate diagnostic log button.
The Lr Mobile app log can be triggered when you open up the settings and long press the to LR Icon a diagnostic log will be generated and attached to your local mail client. While opening the settings could you double check if you are signed-in?