Hello,
Thank you for reaching out!
I've tested a /29 netmask running RUT9_R_00.07.03.2 and it seems to work as intended:
Could you try updating to the latest version and trying it again?
Also, please keep in mind that the non-preset netmask must be entered into the --Custom-- area of selections:
Let me know if this helps.
Best regards,
DaumantasG