I am not an expert in Time systems but I imagine the problem is similar to daylight savings problems. With a leap second it's like putting the clocks forward an hour. With daylight savings, In your time series database you suddenly go from 0059 to 0200. 0100-0159 are missing, but that's fine. Most systems are fine with missing data, because we assume there will be occasional outages and such

When you go the other direction there's a potential problem. You have records with times from 0100-0159, then the daylight savings shift happens and you repeat 0100-0159. Now you have records with (apparently) duplicate timestamps.

I think it's probably the same problem with leap seconds and negative leap seconds, but I'm not 100% sure