BUG: DataFrame.apply not adding a frequency if freq=None (pandas-dev#22150)

Author: HannahFerch

pandas/core/indexes/datetimes.py

@@ -854,8 +854,6 @@ def union_many(self, others):
                 if isinstance(this, DatetimeIndex):
                     this._tz = timezones.tz_standardize(tz)
 
-        if this.freq is None:
-            this.freq = to_offset(this.inferred_freq)
         return this
 
     def join(self, other, how='left', level=None, return_indexers=False,