my .otf export is going well but when I open the exported files I notice a “mark positioning” problem
I can’t fix it !
Glyphs 3.1.1
The error messages you see are not about the exported font being incorrect. Instead, something went wrong when Glyphs imported the .otf file. If you can share the .otf font file with us (email to support), we can have a look at what went wrong and how to improve the import.
This is not something you need to fix. You should not need to open the exported .otf. Work on the .glyphs file. Open the exported .otf only if you want to inspect it, but not to continue working on your font project.
This is a problem with the import of the OpenType features. It is fixed in the latest cutting edge version. But in most cases you like to set up the mark positioning with anchors and let Glyphs generate the mark feature from it. So it is quite save to delete his feature.