Voice 2 lyric melisma disappears

I'm working on a score containing Christmas carols for SATB in closed score (staves S+A, T+B), so there are often differing rhythms between two of the parts. For instance, I have a bar at the moment that contains a dotted minim for the sopranos, and three slurred crotchets for the altos. In order to make the lyrics match the music, I'd normally (in another score writer) put the altos in voice 2, and then enter lyrics into voice 2 with underscores to match the length of the melisma.

When I do this in MuseScore 4 (OS: Windows 10 Version 2009 or later, Arch.: x86_64, MuseScore version (64-bit): 4.1.1-232071203, revision: e4d1ddf), the line appears, then the cursor is positioned under the next bar, which generally already contains lyrics for voice 1, so I click away to exit entering the lyrics. At which point the melisma line disappears. If I swap voices 1 and 2, and create the lyrics in voice 1, then swap them back again, the line does stay in place, so I have a workaround, but it's a pain.

I've attached a test case - try to add underscores to the first bar of lyrics, and in my copy of MuseScore, this behaviour removes the line as soon as you click out.

Anyone have any ideas why this happens? Is it just a bug, or am I doing something wrong?