We've added a quick-fix for this issue, so you'll see it working in builds dated December 25 and later - it looks like a bug in our caching of cell-styles is ultimately responsible (the cached class name is badly formed), so a deeper change is likely necessary.
I don't see a change using v12.0p_2018-12-26 but will wait for your update - perhaps the quick fix was 12.1d only or similar. This is not urgent for me.
The initial quick-fix was indeed in 12.1 only - that's been removed now, and a proper fix put in place in 12.0 and 12.1 - please see builds dated January 7 and later
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