I'm not privy to exactly what your situation so I can't say where things are, what exactly you have, or how things are configured in your environment. Before I give you my educated five-minute guess, I'll recommend that you start by calling PTC Support to diagnose the situation and answer your question. There's no charge if you're current on Maintenance (one of the reasons you get it).
My educated guess does allow me to say this:
The FOSI you're using is probably for print output, yes? If someone else provided your print output stylesheet and they are responsible for it, you don't want to monkey with it.
Many people use their print stylesheet to edit with, but it doesn't have to be that way. You can create your own stylesheet for your own purposes: You can create a totally separate and distinct stylesheet that is for use in the editor only. Editor allows you to have a stylesheet for authoring only: we call that the in-editor stylesheet because it changes the way the tags and generated text appear in the Editor.
You can have two (or even more) different stylesheets to serve different purposes. (Truthfully, you can do it all in the same stylesheet, but that's an advanced topic.)
Start by calling PTC Support. You can use your support call for questions as much as bug reporting. That's what you pay maintenance for. If you want another option, you can get my book, Arbortext 101, which explains the basics behind all those files you're looking at in the install tree, at Amazon or you can book a time-limited, budget-friendly appointment with me to do your diagnosis.
Start with Support, though, because their help is included with your annual maintenance.
