A lot of people in the Internet complain about their version of SSMS “forgot” some hot-key combinations. The oldest complain I remember was about the most useful combination “Ctrl-R”.
The reason why SSMS “forgets” is within code sharing and reusability with other Microsoft development products.
If you have that problem, most probably I have (or had in the past) installed something else from Microsoft, such as Development Studio, Data Tools etc.
It can get annoying when another tool clobbers your expected shortcuts.