The source that should be trusted in this case is the one directly from VDOT versus Wikipedia. It seems possible that the VA 124 designation extended up to Lorcum Lane at some point but the Wikipedia information wasn't updated, whereas the VDOT LRS data was updated back in February of this year. The relation still extending up to Lorcum Lane is an oversight on my part.

In Virginia, it's very common for state highway designations to end abruptly at county and municipal boundaries or above or below other highways like I 66 in this instance. Throughout much of the US, road numbers simply imply who funds/maintains the roadways, leading to examples like this. I am not 100% sure why VDOT would want to hand over a short section of an already short route over to Arlington County to maintain, though, but my guess is VDOT is slowly trying to decommission VA 124, effectively making it an unsigned county road, and is now down to the section that is below the I 66 right-of-way.

A similar example of this is VA 228, which starts at VA 7 in Dranesville, bypasses Herndon, then is briefly rerouted onto Elden Street before abruptly termination at the boundary between Herndon and unincorporated Fairfax County, where it becomes Secondary Route 657, a county-level highway. For whatever reason, VDOT sought fit not to continue funding the short section from the town boundary to the Dulles Toll Road.