whatsnew and changes requested

Author: fleimgruber

doc/source/whatsnew/v0.23.0.txt

@@ -1292,6 +1292,7 @@ Indexing
 MultiIndex
 ^^^^^^^^^^
 
+- Bug in :func:`io.formats.style.Styler.applymap` where ``subset=`` with :class:`MultiIndex` slice would reduce to :class:`Series` (:issue:`19861`)
 - Bug in :func:`MultiIndex.__contains__` where non-tuple keys would return ``True`` even if they had been dropped (:issue:`19027`)
 - Bug in :func:`MultiIndex.set_labels` which would cause casting (and potentially clipping) of the new labels if the ``level`` argument is not 0 or a list like [0, 1, ... ]  (:issue:`19057`)
 - Bug in :func:`MultiIndex.get_level_values` which would return an invalid index on level of ints with missing values (:issue:`17924`)

pandas/core/indexing.py

@@ -2733,11 +2733,10 @@ def _non_reducing_slice(slice_):
         slice_ = IndexSlice[:, slice_]
 
     def pred(part):
-        # true when slice does *not* reduce False when part is a tuple,
+        # true when slice does *not* reduce, False when part is a tuple,
         # i.e. MultiIndex slice
-        if isinstance(part, tuple):
-            return False
-        return isinstance(part, slice) or is_list_like(part)
+        return (isinstance(part, slice) or is_list_like(part))\
+            and not isinstance(part, tuple)
 
     if not is_list_like(slice_):
         if not isinstance(slice_, slice):