Too late I am restoring from a backup.
But I can certainly reprhase, I saw the same thing happen twice with this update.
Normally, when you access Components > Gantry 5 Themes, you are presented with all the G5 templates you have installed. You can then pick one to work with.
Twice now, after updating to 5.4.2, this is no longer the behavior. When I access Components > Gangtry 5 Themes, I am no longer presented with the templates I have installed. I am immediately brought into the Callisto template that I am using.
So, instead of this:
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I get this:
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But its not even that, the style presets are missing, and I'm not even sure what else is missing.
If you REALLY want me to, I'll do it again and reproduce (its happened twice, I'm sure it will happen a third time).