Crack along floor Joist in crawl space. Another similar on the same joist further down. Major issue or simple fix? Suggestions?
tl;dr It's probably just a localized weakness and not a serious problem.
I suspect that the joist cracked due to 1) the off-color streak we see, which may be heartwood, and 2) the knot. Both are very weak points in the wood that wouldn't normally be a problem, but their unfortunate proximity combined to dramatically weaken the wood.
The outstanding question is whether there's undue stress on that particular joist, causing sag and severe tension on the bottom edge. We can't say much about that through this little hole in the internet.
I’m always surprised how people can look at a structural failure and determine that it’s “not a serious problem.” I have to calculate the load to determine if it’s serious. YOU HAVE A FAILURE. Do not ignore it.
A 12’ span is not extreme, but those joist seem extra large for normal construction. If they’re more than 2x12 at 16” o.c., then the designer was trying to do something special.
Is there something special above those joist: like a hot tub, waterbed, etc.? Make sure you follow through and have it checked out. At a minimum I’d fix the support for the pipe. That sagging could put stress on other joints.