Quantity Step Is Not Working as Expected
Confirm the effective minimum
The valid sequence depends on both the minimum and the step.
For example, Min 5 and Step 5 creates 5, 10, 15 and so on.
Check inheritance
A blank Step field may be inheriting a global or parent value. Review variation, product and global settings in that order.
Check third-party quantity controls
A theme or plugin may visually change the input increment. The server-side validation should still be tested by adding the product to the cart and updating the cart quantity.