Once you regained the signal you need to hold onto it for 45 minutes to an hour uninterupted, if you lose it before SF2 fallback is ready it goes to waas, I advise my customers to switch to a tower ID that doesn't exist before leaving an area with a strong signal when going to an area that signal may be lost, this way they will have SF2 fallback, if you go into the diagnostics in the receiver there is a SF2 Fallback section that will tell you if it is ready to use or not. 

Not sure why you could not get signal when using the grace period, you can always shut RTK off and go back to SF1 when you can't get SF2 fallback to enable.