When adding a vertical dimension (for small caps), it does not stay from session to session.
Are you using a version 2 format file? See the Other tab in the Font Info window.
I am using Cutting Edge version 3
Your Glyphs version is 3, but Glyphs 3 can edit Glyphs 2 compatible files which do not support custom metrics. Check the Other tab in Font Info to see what file version you are using.
I was not aware that Glyphs 3 could save as Glyphs 2.
That worked, now the measurement stays. Thanks Florian!