If it happens exacty when you shift to 5th, and stays on as long as you're in 5th, and it still works in neutral, like it should, and at no other time; and given what work was done, I'm guessing that the clutch may be a tad out of adjustment. I'd try that, and while you're down there, you may as well remove the neutral indicator switch from the housing and make sure it doesn't have somethign stuck to it, or something. The bike must be cold to adjust the clutch. Follow these steps for the clutch adjustment:
1) Induce maximum slack in the cable by loosening the jam nut then the adjusting nut inside the rubber boot on the cable.
2) If you need to lube the cable, remove the cable end from the lever by removing the snap ring, and pushing the retaining pin up through the lever bracket.
3) Remove the clutch inspection cover, using a “star pattern” on the five retaining bolts. Note which hole on the cover is to the top and replace it the same way (the cover eventually takes the shape of the primary cover, so this prevents leaks).
4) Loosen the 11/16” nut in the center of the clutch.
5) Using an Allen wrench, loosen the clutch adjusting screw, which is the center of the threaded rod inside the nut.
6) This is the critical step. Holding the Allen wrench between your thumb and forefinger, and tighten it (turn it clockwise) just to the point that resistance is felt. From that point, turn it counterclockwise ½ to 1 turn (1/2 for most riders is good, ¾ if you use the clutch a lot for control at low speeds, a full turn I use only for police bikes).
7) Making sure to hold the Allen wrench at that position, tighten the clutch nut. If you have a crow’s foot, you can torque it to spec, which –12 ft-lbs. – if not just get it good and snug. An offset wrench is best, but you can use a regular open end wrench. However, if you do, be careful not to let it slip off the nut, since it will be at an angle to clear the primary cover.
8) Replace the clutch cover, and gasket, tightening in a star pattern to the right torque (84-108 inch/pounds or 10 n/m).
9) If you removed the cable end from the lever replace it, and tighten the cable. Squeeze the lever three times to re-set the ball and ramp mechanism in the release. Then fine tune your cable adjustment, so the free-play is correct. (The edge of a nickel should fit between the shoulder of the ferrule and the bracket.
They may have done the adjustment with the bike warm. THat could throw things off just enough so your getting a little engagement, which might cause the switch to sense you're in neutral. Of course you should recognize that, with all the "mights" and "ifs" in what I've written, I'm about one step away from making this up as I go. However, the clutch adjustment is the only thing in what they did that could possibly have any bearing on this, so I'd try that. Plus it only takes about 10 minutes, so you don't have anythign to lose. You can skip Step 2, since one hopes they lubed that cable for you.
The mechanic that maintains the H-D Police training fleet swears a student called him over as the motor was waming up and said "The compass is broken". He asked "what are you talking about?". The student said "Look the 'North' light is on and the bike is facing west."