In fact, this behaviour is very common for NList-based listviews. The 'problem' is, that NList is per default automatically adjusting the size of each column depending on their content width. So if you have a mail with e.g. very long subject lines or if the status column contains a changing amount of status icons, it may happen that NList automatically decided to resize the columns automatically.

While this can be very annoying during normal work, it can also proof quite usefull in some cases. However, there is also the possibility to force NList not to automatically resize a column. For this to happen, you have to drag the vertical separator to the desired size (you actually have to drag it at all) and use the MUI 'snapshot' feature you can normally access via one of the top-right border buttons in the YAM main window.