Empty tabs (about:newtab) are removed from tab groups when the groups are saved and closed. Bug or feature?

Not sure if it's a feature or a bug, but I noticed that if I 'save and close' a tab group containing an 'empty' tab, i.e. the about:newtab page, the tab does not get restored when the group is opened again. Before I was using tab groups, I often used these empty tabs as a crude method of separating bunches of tabs from each other. I have some large tab groups, open on the tab bar, that still contain these, as I have never closed them since they were created. (They're just collapsed.) I was planning on closing them sometime, after I'd next reviewed their contents, but now I'm not too keen on that! So, just wondering, is this empty tab deletion a bug or a feature? If the latter, what's the rationale?