We lasso select these buildings, producing:

And then we type "C" to merge them.

Everything is going well,

Except well, they have disappeared from the map,

and the left panel shows

Changes (11)
Deleted Building
Deleted Building
Deleted Building
Deleted Building
Deleted Building
Deleted Building
Deleted Building
Deleted Building
Deleted Building
Deleted Building
Deleted Building

I mean we were taught in school,

According to the law of conservation of matter, matter is neither created nor destroyed

Or in corporate mergers, shareholders shouldn't be left with nothing.

Well @jidanni, what do you expect / want in this case?

I don't know. I was just doing an experiment to find out, but "never expected the worst to happen". At least there should be a warning printed.

Come on, a busy person like yourself, you certainly had something you really wanted to accomplish.

Well OK, but is has nothing to do with this bug report.