This is a known bug in the current release. It will be fixed in the next release.
Ctrl-G and Ctrl-Shift-G should move you back and forward on a find (Ctrl-G is Find Next/Ctrl-Shfit-G is previous), but as Ric noted this a known bug (currently fixed internally). There are two other ways to do this. One as you noted is through Previous and Next on the Reading Menu. The other is to use the two buttons next to the right of of the Find control of the toolbar.
However if all you want is just keyboard navigation through the text there are quite a few other key combinations documented on our help page. One of the more useful set for reading is scrolling up or down by a page, which can be down with the right and left arrow keys respectively.
I've checked all available key combinations as documented in the help page. All work in correct manner in "Fit Single Page" mode. While using "Fit To Width" or "Custom Fit", the key combinations simply can't provide a correct page flipping behavior. Each key press sets you in a different location in the next page.
As the "Fit Single Page" mode provides you with rather small fonts, I'm trying to avoid this mode.
The only method I find acceptable is to use old fashion vertical scroll bar with mouse.