"The imbalance is tiny, just trust the lift", why not?

An experiment intended a 50/50 split, but the SRM check fails badly (chi-square far above the threshold): control got noticeably more users than treatment. The dashboard still shows treatment beating control by a clean, "significant" +4% on the primary metric. A colleague argues: "The imbalance is only a percent or two, and the lift is big and significant, let's just ship it."

Explain why a sample-ratio mismatch invalidates the result even when the lift looks strong, what typically causes SRM, and what you should do.