Seems like such a “feature” would introduce risk of blocking the ability to autofill the master password with a different password manager (such as the forthcoming desktop auto-type feature). Blocking such capability would technically run afoul of an important password standard:
NIST 800-63B
Verifiers SHALL allow the use of password managers and autofill functionality. Verifiers SHOULD permit claimants to use the “paste” function when entering a password to facilitate password manager use when password autofill APIs are unavailable.