This is called “layering”and can be a problem by 1) lack of bonding, and 2) segregation of materials.
1) Bonding can be a problem, as you pointed out, allowing moisture between the pours and then freezing. This will cause leaks and possibly spalling.
In addition, and is usually a larger problem, it allows moisture to affect the rebar and cause rust. When steel rusts, it expends and can deform the wall.
2) By not compacting the two lifts together, you have “two walls”, one on top of another with a HINGE between them. Compaction prevents honeycombing and assures close contact with forms and rebar. Rebar is only effective if it is bonded to the concrete. The line is an indication of poor workmanship and thus poor bonding.
Unfortunately, it’s just unsightly now and difficult to prove inferior and needs to be replaced. Give it 20 years and it will really be an issue.