The "Start Active Players" button should be able to swap bench player for injured player, if pressed when a player is listed as "Questionable".
See my CHAT discussion, copied below:
"Start Active Players" button, does not work.

On Monday 9/21, I had Puka Nacua questionable. I moved Nacua to the Flex position and picked-up TE Terrance Ferguson on Waivers for my bench. I then pressed the "Start Active Players" button a few times on Sunday and Monday. But In the game, Nacua wound up staying in the starting position and Ferguson was on the bench, with 17 Fantasy Points.

I am usually not available on Sunday and Monday nights to monitor and adjust to game time decisions. So the "Start Active Players" button is important to me. Does the system need a fix, or am I not using it correctly.Type a message

Website on laptop. Button seems to work when pressed, but did not affect lineup at game time
Yahoo: check mark means the Start Active Players request was accepted, but it does not guarantee every eligible swap occurred.
For the injured Flex player to be replaced, the benched player needed to be eligible for that Flex slot and both players needed scheduled games. The injury status is also evaluated when you select Start Active Players, not later. If the starter was marked injured after you used it, the tool would not account for that change.
Were both players scheduled to play that day, and was the bench player eligible at Flex?
Me: Bench player eligible at Flex. Starter was "Questionable" when Start Active Playersselected. If it only works when starter is marked injured, it is NOT USEFUL for the purpose it should be intended for
Yahoo: You’re right that a Questionable designation does not meet the tool’s “injured” condition. Start Active Players only replaces a starter when the starter is marked injured and both players have scheduled games; it does not make a swap based on a Questionable status.
Me: NOT USEFUL for purpose it should be used for. If I was online when the player was listed as injured, I could have made the change myself.
Yahoo: That’s fair feedback. The feature is designed for limited automatic lineup swaps, but it cannot anticipate a player changing from Questionable to injured after you select it. Because lineup changes are locked at game time, it may not help in the situation you described. For a player listed as Questionable, the practical option is to manually move an eligible bench player into the Flex spot before that player’s game begins.
ME: I did move the Questionable player to the Flex spot before the game, and had a Flex eligible player on my bench.
Agent said
That setup was appropriate: placing the Questionable player in Flex and keeping a Flex-eligible player on the bench preserves your manual flexibility before kickoff.
However, Start Active Players evaluates injury status only when you activate it. It does not make a later swap when a player’s status changes after that point, even if the player is later ruled out before the game. The check mark confirms the setting was accepted, not that it will react to later injury updates.
You said
"Preserving Manual Flexibilty" is USELESS
Agent said
I hear you. In this situation, it did not provide the protection you expected: the feature does not monitor a Questionable player and automatically replace them if they are later ruled out. It only evaluates the lineup at the time you select it, so the check mark was not an assurance of a later injury-based swap.
You said
NOT HELFUL OR USEFUL. I am very disappointed.
Agent said
I’m sorry this feature let you down. Your expectation was reasonable: with a Questionable player in Flex and an eligible bench player available, you expected the setting to cover a late inactive update. Start Active Players does not do that, and the confirmation check mark can make that limitation easy to miss.