Problem with ties

I'm not sure whether this is a bug, or a new "feature", but without warning, it seems that some sorts of ties have stopped working. In the example here, I want some of the semiquavers on the first beat of each measure in the left hand of the piano to be tied, rather than restruck on the second beat, but MuseScore is refusing to do this. It's ALWAYS worked before, so what is happening here? I am totally mystified.