Looks like you got stopped out at 10:05 Central with the low hitting 434.85 This chart is Mountain Time
It looks like Think or Swim must have had connectivity problems with the data server for the charts, but the trading servers executed properly. The intraday data looks like it went offline around 10:55 Eastern, tried to reconnect briefly at 11:45 Eastern and again at 12:50 Eastern, and finally reconnected for good at 13:00 Eastern. So you have no real beef with Think or Swim other than the front end chart missing data. You orders were executed properly and timely. It looks like SPX and ES data at Think or Swim was unaffected. So a work around for you would be to have both an SPX and SPY or ES and SPY chart to watch. If the data diverges, you will know you have a problem. I don't day trade at Think or Swim but I do trade SPY there and I have not had any problems with execution of orders either.
You need to have faith in your trading strategy and market data, order execution. When I went long, market was already below my stop loss. So, it was a losing trade from the beginning. Maybe, this is why day traders lose in the long run or better market data may help them win. One of these statements is true.
But you didn't get stopped out until 10:05, so the trade wasn't already a loser at the time of entry.
SELL -1 SPY STP 434.90 [TO CLOSE] 9/28/21 09.59.08 Don't know what you are talking about. I clearly said that I was stopped out at 09.59.08 central. But at that time, my chart was showing 09.55am price which was 435.24. Looks like market data was interrupted between 9.55am and 10.45am.