Add tied note by keyboard fails when notes are already present

Scenario 1 OK:

Empty measure
Keyboard:
5
c
4
+

Expected result = actual result = ok = 1 quarter C tied to a 1/8 C

Scenario 2 failure:

measure with already a quarter C on beat 2, cursor on beat 1.
Keyboard exactly as scenario 1:
5
c
4
+
Expected result: same as scenario 1 (plus a 1/8 c on second part of beat 2)
Actual result: 2 tied quarter c :-(
test.mscz