The 28-day rule is mainly about whether the required funds were held for the full evidence period, but visitors are also assessed on overall credibility and consistency.
Timing errors, unexplained deposits, and mismatched bank evidence can make a visitor application look weaker even when money is present in the account.
Before paying government fees, many applicants use the UK Visa Risk Checker to review financial credibility.
Sources: Immigration Rules, official Home Office fees, and public migration statistics.