VBox immediately after HBox in pattern of Measure(s),HBox,VBox,Measure(s) does not split system

If start with any number of measures, followed by a single HBox, followed by any number of measures, e.g.:

After inserting a VBox before that measure after the HBox, the resulting layout incorrectly shows no visual change:

However, the debugger clearly shows that the VBox has been inserted, even though the visual layout does not reflect that fact:

Turns out system layout was not adding an implicit line break when enountered VBox right after HBox in this situation.