In the font file, there is a very high number of kerning pairs, which causes Glyphs to add a high number of subtable breaks. I was able to export it by adding a Use Extension Kerning parameter to File > Font Info > Font > Custom Parameters, and removing the subtable breaks from the features.fea file in the Temp folder and regenerating it by double clicking generateFont.command.
Script > Open Scripts Folder and you will find the Temp folder next to the Scripts folder. Inside, you should find the temp folders of your exports. Perhaps it makes more sense to downgrade to 983, or to wait for the upcoming cutting edge version.
Thanks for the heads up George. Do you by any chance know when you would have the fix? I found the feature.fea file as Mekkablue pointed, but I still have no clue what the subtable breaks are and how to remove them. lol. So I’m counting on your new fix at this point,
I had a look at it. The problem is that you put the slash in the same group than the Delta. That messes up the sorting of the pairs. I added a filter for that. This will be in the next update. But for now, you could remove the kerning groups from the Greek letters - they are not useful anyway as you don’t have the rest of the Greek letters. Maybe you rename them to be math symbols. Or delete them.