Maybe someone else encountered this, but when I went on to Halo Waypoint (Website) I checked my Halo:Reach achievements under my Service Record and it said all of my achievements are unlocked. The website also said I was on milestone 1 (no progress made to next milestone) when I checked under Career Overview. On the Xbox 360 the Waypoint app says I have milestone 29, I did acquire the achievements in Halo:Reach and it is still shown when I check my achievements on the Xbox. I don’t know if you guys know this is happening, but I’m just stating the bug I encountered.
The whole milestone thing has been buggy the last few weeks. Mine has done the same thing, they should have it fixed before Halo 4 no worries.
Weird, I just loaded up the Waypoint app on my Xbox and my milestone is completely wrong. It used to be really high but now its displaying 25 even though I have all achievements on Halo 3, almost all on Reach and most on ODST and CEA. Weird, it still displays all my friends with the correct milestone number when I go on player card compare. Hope this is fixed soon 