Titanium's vaunted high tolerance for strain, and it's resultant very high fatigue limit, is "balanced" by it's higher sensitivity to manufacturing error.

Quite a few makers of titanium frames have had issues with the welding purity, many of those makers (besieged with warranty and perhaps liability claims) went away early on during "peak titanium".

Obviously Litespeed has (on average) had a good handle on managing the manufacturing difficulties as well as the design limitations having to do with titanium's other characteristics (such as stress concentration limits), which leads me to believe that the OP's frame has endured a decently-long service life under a not-so-casual rider.
It's not just the size and power output that counts though, since even ex-racers tend to put their bikes through tougher riding situations (such as speed over rough ground) than non-racers.

That the failure started at the highest-stress location along the downtube just makes it seem less likely to me that the failure represents a serious manufacturing lapse.
Some welds do perform better than others though, even among large samples of welds produced to high standards.
And as well, compared to some boutique brands, Litespeed frames are "made to a cost" relatively speaking.