@danielgibert This thread started by @LucasFonts back in May is probably the most relevant:
https://forum.affinity.serif.com/index.php?/topic/204670-variable-fonts-kerning-broken/

I’ve commented with an illustration there for good measure.

On the latest official bug list (https://forum.affinity.serif.com/index.php?/topic/220123-26-beta-builds-bug-fix-list/) there is “Custom Variable font is using the wrong kerning value in Affinity [AF-3768]”, but I’m not sure if that is the same issue or not, and the linked bug ID just leads to an empty search results page.