This appears to be a rather complex permission issue. ¯\_(ツ)_/¯
I've upgraded to the latest version, 15.5.1 build-15018445
I have multiple logins with varying credentials for the machine I need this to work for.
If I login as my standard/non-admin user, launch Workstation, Virtual Network Editor -> Change Settings -> Enter admin credentials, I am still limited to 2 octets.
However, if I login as admin, launch Workstation, Virtual Network Editor -> Change Settings -> verify in the popup that this is allowed, I am able to enter the 3 octets as expected. This issue is resolved for me as I can set this up as admin and use as by standard user. But a very curious situation that the entries are limited in my scenario...