I've been using uVision for a couple months now (V5.15) and have experienced this bug that is extremely frustrating.
The bug is that after using the Forward/Back navigate buttons, eventually, I get stuck so that forward movements (e.g. jumping to a definition) no longer register to the navigate backwords button. The button is greyed out and there is only the option to navigate forward. No amount of movement will reactivate the back button at this point and the only workaround is to click the navigate forward button a bunch of times till I am as far forward as possible and then forward movements begin registering in the back button again (but will eventually fail again at some point).
I don't know what exact Forward/Back navigate buttons you talk about, but I recommend to update your MDK installation to our latest release V5.21a and to then compare for differences, as many bugfixes, feature requests and enhancements are included already