Although the forum allows names with spaces - eg, "Einar Thorsrud" - the automatic lookup when you type the '@' to mention someone does not work with spaces!
This seems to be an inherent fault in the underlying forum software.
ARM have just started using what appears to be exactly the same product for their Keil forum - and it has exactly the same issue!
The problem is compounded by the fact that the list of suggestions seems to be just a random selection from all the names which contain the string in any position.
Here's a Keil example:
So the list would be a lot more useful if it prioritised:
Again, this looks like an underlying flaw in the 3rd-party product - so, presumably, outside Nordic's control?
But perhaps you could pass this on to the supplier?