FIX: Fix add_reference_channels for passing two channels names (#13466)

Author: mistraube

doc/changes/dev/13466.bugfix.rst

@@ -0,0 +1 @@
+Fix bug with :func:`mne.add_reference_channels` not working correctly when passing more than one channel name, by `Michael Straube`_.

mne/_fiff/reference.py

@@ -316,7 +316,7 @@ def add_reference_channels(inst, ref_channels, copy=True):
             "unit_mul": FIFF.FIFF_UNITM_NONE,
             "unit": FIFF.FIFF_UNIT_V,
             "coord_frame": FIFF.FIFFV_COORD_HEAD,
-            "loc": ref_dig_array,
+            "loc": ref_dig_array.copy(),
         }
         inst.info["chs"].append(chan_info)
         inst.info._update_redundant()

mne/_fiff/tests/test_reference.py

@@ -744,6 +744,10 @@ def test_add_reference():
     ref_data, _ = raw[[ref_idx, ref_idy]]
     assert_array_equal(ref_data, 0)
 
+    loc1 = raw.info["chs"][ref_idx]["loc"]
+    loc2 = raw.info["chs"][ref_idy]["loc"]
+    assert loc1 is not loc2
+
     # add reference channel to epochs
     raw = read_raw_fif(fif_fname, preload=True)
     events = read_events(eve_fname)