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The mortice saved you but aren't there some regulations about not having a key operated lock on a communal door? Perhaps it's only on new builds or something, anyway, based on this, I'd just keep schtum and carry on using it.
Fire regs allegedly, although I'm finding conflicting info if it is a secondary lock (if it's the primary lock it has to be unlockable from the inside without a key).
Some mortice locks already can do either (key or thumbturn) it just depends on how it has been installed; some are key only.
Similar position myself as we share a outer door with the flat upstairs and that has a key only mortice lock on it.
I'd guess the thinking is that if they could get through the front door quickly they would be able to set to work on the flat doors with a bit more time and without being seen. Although more likely is there isn't much thinking going on at all, just give it a go and see what happens.
The mortice saved you but aren't there some regulations about not having a key operated lock on a communal door? Perhaps it's only on new builds or something, anyway, based on this, I'd just keep schtum and carry on using it.