I would turn off JS minification. I've seen bugs around that feature that resulted in the same symptom back in 4.1.x and I think in 4.2.X, so it's worth a shot.
In $SPLUNK_HOME/etc/system/local/web.conf, in the [settings] stanza, put
minify_js = False
if you have no etc/system/local/web.conf, then create it, and have its contents be:
[settings]
minify_js = False
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