Composed Looks: Custom fonts and colours not applying with custom master page

This is on SharePoint Online.

I have a custom spcolor file, a copy of an existing file with my colour changes made. I also have a custom spfont file, where I've replaced all font definitions with 'Calibri'.

In addition, I have custom master page (and preview) which at this stage is identical to seattle.

I have uploaded all the files to the relevant libraries and created a new composed look item.

The problem is that if I attempt to use the custom colour or font definitions with my custom master using the 'Change the look' function, they do not get applied and the 'Try this out' preview reverts to the default fonts and colours. I can use my custom colour or font definition if I use the seattle or oslo master pages.

I did come across a single mention somewhere that Composed Looks only work with seattle, but cannot get any verification (and they work with oslo, anyway).

Edit: After verifying that it works correctly in 2013 (Azure dev instance), I tried again with the only difference is that I used the "Download a copy" link for getting a copy of seattle and it appears to be working fine. I'll leave the question open in case someone comes up with a technical reason for this.