This can be solved by ensuring that your swimlane logic is explicit in setting the criteria. I am not sure on the exact logic used but it does appear to be somewhat random.
Although tedious, try setting the Swimlane as follows to test
Swimlane 1 - Critical
- Severity = Critical
- Severity <> High
- Severity <> Medium
- Severity <> Low
Swimlane 2 - High
- Severity <> Critical
- Severity = High
- Severity <> Medium
- Severity <> Low