Beam across a system break can disappear
In V3.0.4, the fix for the #280050: Beam Middle not working across barlines problem gives a major improvement for the long standing #16278: Beaming notes over barlines and line breaks problem BUT there is a display / export issue. With the attached score (in Page View), enter a note in the first empty bar and the beam across the previous line break disappears.
The fault may correct itself on saving / moving the window and other operations, or it may not, but it is not very persistent. However, exporting a pdf while the fault is displayed, will export the faulty score.
Although breaking the beam at the line break is MUCH better than having fragments of beam wandering over the page, this is, however, not a real fix for #16278: Beaming notes over barlines and line breaks.