It shows interaction between fixtures and should be reported as such. The shower symptom may not be confined to its own visible waste fitting.
What the observation means
A shared route, pump or installation issue needs assessment. Do not repeatedly run the basin to reproduce the return flow.
A useful next step
Stop using the affected connected fixtures and describe which one was used first. Keep contaminated areas out of use. Do not cap an overflow, block the shower drain or alter valves to force the water down another route.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.