Just to add more confusion: I have a copy of that same lens (SMC Takumar 3.5/28) and the mirror *does* hang on my 6D. Not only that, but it does hang quite a bit, or in other words, it does take a fairly significant amount of rotating away from infinity to free the mirror.
And while I'm at it, I might as well add more info about other m42 lenses I have.
I found the following lenses to cause the mirror to hang on my 6D at infinity:
- SMC Takumar 3.5/28mm
- Super Takumar 3.5/35mm
- Carl Zeiss Jena Tessar 2.8/50mm
On the other hand, the following cause no hanging whatsoever:
- Meyer Optik Oreston 1.8/50mm
- Super Takumar 1.8/55mm
- Helios 44-2 2/58mm
- Super Takumar 1.9/85mm
- Avanar DC 2.8/135mm
- Tokina RMC 2.8/135mm (that one with CY mount, not m42)