The register's far-right Cash balance column carries End-of-day core cash: the account's whole core-cash pool, settlement cash plus every effective-dated cash vehicle, as of the end of a date, over the account's complete recorded history. It lands on that date's chronologically last visible row; every other row of that date leaves the cell blank, so nothing on screen reads as a per-row running balance.
It is never a sum of the rows shown for that date, and it is never floored to zero. Filtering the register down to one security, one activity, or a date range changes which rows carry a figure; it never changes what the column reports, which is why a filtered view still says Cash balances include all recorded account activity. It is also a different figure from the value chart's smoothed cash, which spreads a trade's settlement over a few days for a readable line: this is an exact daily balance from the ledger, not a chart point.
On the marked row, a balance that could not be computed for that date shows an em dash rather than a misleading zero, with the reason in the cell's tooltip.
The column shows only when the register is sorted by Date. Sorting by Activity, Security, Shares, Price or Amount blanks it, along with the Share balance column beside it, and one line above the table offers Sort by date to bring both back. The balance hasn't changed; only its place in the re-sorted list would be misleading to show.