Why did my prop-firm account breach when I closed the day in profit?
Because the daily loss line is checked on equity at every moment, not on the day's closed result. If equity touched the line at any point, the account breached then, and closing later in profit does not undo it.
The rule is a floor equity may not touch during the day. A trade that goes 3,000 against you and comes back to close 400 in profit has, on the way, put equity 3,000 below where the day began; if the line was 2,500 below, it was crossed on the way down. FundingPips answers exactly this question on its own help page and the answer is that the account is violated. FTMO's rule is the same measurement.
Two things make it happen to careful traders. The first is the base: at FundingPips the day's base is the higher of opening balance and opening equity, and at FTMO it is the balance at midnight, so a day that opens with a floating profit has less room than the balance suggests. The second is a second position: two trades each inside their own stop can together put equity past the line, because the line is measured on the account, not on the trade.
A breach is detected by the firm's server, on its own feed, at its own tick. A trader who saw the low on their chart at 2,450 and the line at 2,500 can still have breached on the firm's price, which is why the firm's own dashboard figure is the one the rule is measured on.
Worked example
FundingPips, 100,000 account, opening balance 100,000 and opening equity 101,200 from a position carried overnight: the base is 101,200 and the line is 96,140. The position gives back its 1,200 and a second trade opens; together they reach 4,900 under the base, equity 96,300. A spike takes it to 96,100 for two seconds. The account is breached there, and the 700 profit both trades close with an hour later changes nothing.
Where this is from
A firm or a platform changes its terms without telling anybody here. Each firm page links the firm’s own rule page and shows the date it was checked; the firm’s own page is the one that counts.
Daily loss calculator shows this for your own numbers.