Can previously obtained bookmark caches be viewed when connecting using an IP address not permitted under folder IP restrictions?

Yes, you can view the caches.
However, this is predicated on the bookmark caches having been obtained using an IP address that was permitted or prior to the folder IP restrictions being applied.

Related: View manuals offline (Mobile App)
https://teachme.jp/8/manuals/3204796

If you only have bookmarks without obtaining the bookmark caches,
while the bookmarks will be displayed, attempting to view the manual will result in the error "Failed to load the manual" and you will not be able to view it.

*When group IP restrictions are in place, access to Teachme Biz using an IP address that is not permitted is not possible.