autotuning/autoswap
the autotuning (which can be observed on the "Tune" page) can get kicked out for a number of reasons like setting DME hold or switching to green needles. However, the FMS is still scanning for VOR/DME stations on two other "invisible" channels.
The autoswap works only for ILS approaches provided that the approach is loaded in the FMS, "approach" is punched and you are within 90 degrees of the inbound course (you could in theory use the autoswap also for a LOC approach, you just have to remember to switch back from APPR to NAV when the localiser is captured). Normally, the localiser frequency as well as the inbound course should be selected automatically by the FMS and the frequency also should show on the PFD. You also get a "ghost needle" showing the inbound course of the localiser and a message "Loc will be tuned" on the CDU. The autoswap occurs, I think, when the next waypoint is either the FAF or the FACF (a point created by the FMS 2-3 miles before the FAF to ensure a proper lineup of the aircraft before the FAF). Maybe punching DME hold before the LOC frequency is automatically selected prevents the automatic tuning by the FMS? Anyway, there is no harm doing the nav setup manually, it keeps you better in the loop.