Nikon D750 Time mode

Hi,
Is there some kind of limit on the Nikon D750 Time mode?
I'm talking about the mode when you turn the shutter speed past bulb to time. Is there a time limit?

I do astrophotography and star trails and I want to leave the shutter open for an hour or more but it seems that the shutter automatically closes and finishes the shot about 40 minutes or so in without me pushing the shutter button the 2nd time.

That's way too long, instead consider stacking multiple images together and combining in PS.

Stacking honestly takes forever on the computer. I've done hour exposures or more on my D750 and the photos come out great. I just want to know why the time mode automatically closes the shutter after 3o to 40 minutes. You can get rid of the "heat noise" very easily it lightroom and photoshop.

I wonder if it's tied in with the 29:59 maximum video length that some DSLR's are inflicted with

Possibly a heat issue?