Hmm, well I guess it should always run on a build bot.
At the command line if you build once, then build again without changing any files, the mksnapshot step re-runs.

Could you provide cmd line steps? If I e.g repeatedly do:
ninja -C out.gn/ia32.release d8

The output is:
ninja: Entering directory `out.gn/ia32.release'
ninja: no work to do.

Assign to Michael to provide more details.

Okay, this means it only happens on my machine, which is good (or at least, better than it happening systemically). I'll follow up Monday.

Problem went away by itself.