I've recompiled HISE and now my sample map appears to be loading without issue.
Turns out I'd moved some samples which is why it loaded. Issue is still there.
But this has lead me down a little route. @Christoph-Hart Is there a limit to the maximum number of samples you can have in a single sample map?
I'm finding if I add one additional sample (even a renamed duplicate of a sample I've already loaded) then the sample map goes all glitchy as described above. If I remove the sample then it loads.
It seems to be occurring once the sample map contains around 8160 samples. Could it be a buffer issue?