Using Shift+Arrow: 
goes: 100, 90, 80, 70… 20, 10, 1, -9
Expected: 100, 90, 80, 70… 20, 10, 0, -10, etc
Also, after this happens, I can’t go back to Zero:
I enter 0 and then press Return, but Glyphs changes it to 1
latest beta Version 2.6.5 (1308)
Using Shift+Arrow:
goes: 100, 90, 80, 70… 20, 10, 1, -9
Expected: 100, 90, 80, 70… 20, 10, 0, -10, etc
Also, after this happens, I can’t go back to Zero:
I enter 0 and then press Return, but Glyphs changes it to 1
latest beta Version 2.6.5 (1308)