Anyone got any ideas?
This happens because flowchart topics are actually pre-formatted floating topics joined by relationship lines. MindManager can put a boundary only around a "conventional" topic and all its sub-topics, and not a group of floating topics.
There are however a couple of workarounds. The first is to use the shapes feature added in MM2018 to draw a boundary-like box around the topic group. These are a little inflexible however in that they are static and can't be attached to a topic, so if you change the layout of the flowchart you will have change the box boundaries manually. The range of shapes is also limited to rectangles, circles and lines.
The second is to get around the flowchart formatting to convert the topic into a conventional one and then attach "normal" sub-topics. To do this right click on the topic and from the context menu select Format Topic, then click on the Subtopics Layout to choose the layout you want. Add the elements required as sub-topics then click on the parent topic and add a boundary.
I've described the process in a bit more detail in a response to a similar query about 12 months ago: https://community.mindjet.com/mindjet/topics/cant-create-1-boundary-for-multiple-concepts (see my second rather than first response).
Bear in mind that this process has some limitations. As the parent topic and sub-topics are now conventional rather than flowchart topics it appears to be impossible to add any new flowchart links to them, though any previously-existing flowchart links to the parent topic seem to be retained. In other words it's probably better to do this after you have largely completed the whole flowchart and to do it only with topics where the subtopics are dead-ends not linked to other flowchart objects.
Not only provide they the insight you need, but also help you avoid clutter etc.
In MM2018 the use of swimlanes has improved since they were first introduced.
You find swimlanes under Insert tab and the Objects section.
If you want more... right click on the line and you have the option to change the nr of lanes.