I have already downgraded back to .53, as I found the beta a bit unstable in general use. Checking it out with the debugger, in .53 I can confirm the plugin does get SERV_LEARN, but it also seems to be working correctly and learning from the addresses when I test it here.
If you are finding it does not learn for you, please ensure you are using Misnamed Filter 0.5, and Spamihilator 0.9.9.53. You can check that there isn't some problem with actually saving the settings file by going to the Spamihilator settings window, then on the Plugins page clicking Misnamed Filter and clicking the Configure button. Any errors in accessing the configuration file will be reported before it shows you the Configurator for Misnamed Filter.
In the Configurator window, check that "Learn names for addresses from non-spam emails automatically" is checked. If it's unchecked, Misnamed Filter won't learn anything. Otherwise, click the Test button in the bottom left. From the email that you expected to learn from, that didn't work, copy from the To header the email address (just the part inside the <>) into the email address box, and the email name (the part inside the "", or to the left of the < if no " marks) into the Name box. If the Matches box underneath is not empty, and contains no red text, then the reason it isn't learning from the email is because it's already a valid address-name pair according to your existing settings.
If the text Matches box is empty, or red, and the Learn box is checked in the Configurator, and there are no errors when launching the Configurator from Spamihilator's settings window, and you are using the correct versions of Spamihilator and the Misnamed Filter, then there is a bug. For me to investigate it, I will need a copy of your Misnamed Filter.settings.xml file (should be in "%APPDATA%\Spamihilator\plugins\Misnamed Filter"), and the full headers for the email address you were expecting to learn from. You can email them to me at alex -at- vallat -dot- name