In the phpbb_styles_template table, the current default value for the bbcode_bitfield column is set to kNg= (ids 0, 3, 5, 8, 9, 11 and 12 IIRC), which means that simple bbcode tags (such B, I, U for example) will not be managed by the style by default.
It may cause problems for style authors who don't understand quite well the template bitfield thing and who change the HTML replacement for those tags not managed by by default (and therefore they won't understand why their HTML code doesn't work...).

Thus I suggest that you set the default value to the one containing all default BBcode ids, i.e. //g= (ids 0 to 12).