Recalling a saved registration will restore all settings that were saved, which normally includes the style-related settings-- style number, style variation, and tempo. That means the style can change unexpectedly while you're playing if you recall a registration that has different style settings than the settings you started out with.
The solution is to disable the style settings before you save the registration so they won't be written with the other registration data. To do this, set up the voices and other settings the way you want them to be in the registration, but before you save the registration you must press the SONG button. The display will change to show the currently-selected Song, but you can just ignore that because the song-related settings are not saved in the registrations; the important thing is that you've switched the keyboard into Song mode, which disables the default Style mode. While the SONG button is lit up, save the registration to whichever Bank and Memory you want it to be stored in. Then you should be able to recall the registration without inadvertently causing the style to change in mid-performance.
To recap:
(1) Set up the voices, effects, and other settings as desired.
(2) Press SONG.
(3) Save the registration to the desired Bank and Memory.
If you already have saved registrations that you want to convert to the style-free format, do the following:
(1) Recall the registration to be converted.
(2) Press SONG.
(3) Save the registration again to the same Bank and Memory.