How to add wipe luma for custom video size in kdenlive

I know this is not a kdenlive forum, but there are so many experienced people here that it might make sense to post my question here.

I am trying to work around the nasty bug described here: https://bugs.kde.org/show_bug.cgi?id=423619. The problem is that the different wipe templates (called lumas, b/w images in pgm format) are only used for some project resolutions. The resolution I require is 854x480 (don’t ask, I got the material in that resolution, but at least it’s 16:9), and all the wipe transition options vanish for that resolution. I was not able to figure out after which rule the lumas are selected. When I spread my custom pgm with the required resolution in my local lumas folder as shown below, I see the cloud2 transition for other resolutions, but not for the one required.

~/.local/share/kdenlive/lumas
├── cloud2.pgm
├── HD
│   └── cloud2.pgm
└── PAL
    └── cloud2.pgm

Any hint how kdenlive selects the lumas and how I could fix my issue are very welcome, unfortunately I even cannot figure out where in the code the selection is performed.

kdenlive 20.04.1b appimage in case this is relevant, as newer versions do not output preview video on my rusty 16.04-ubuntu.

I dunno but that size video is FWVGA or 480p.