A baby with mild tt can get frustrated at the breast and fall asleep before they're tummy is full. As bm is digested more quickly than ff they sometimes wake soon after wanting more. (Breast compressions can help, best is sorting the match and tt)
Ff is easier for baby as they simply don't have to do as much work (but doesn't help to strengthen the tongue or flatten the high palate that comes with a mild tt). So they might be full as well as taking a little longer to digest the milk so might sleep a little longer.
When a baby falls asleep too soon on the breast, the breast gets a signal that that's enough. If a baby kept going or was more efficient due to a more agile tongue and better latch/ flatter palate, they'd stimulate more milk production. And the breast makes more milk (hence power pumping can help stimulate supply).
Switch to ff and the breast doesn't get those signals so supply and demand dwindles and stoops.
The ff makes babies sleep can sometimes work but only in the first few weeks, as long as there are no other issues simply because it's harder to digest (I was led to believe, Happy to be corrected) but the effect lasts no longer than this and over all in large scale comparisons between bf and ff there isn't much difference and none beyond the first few weeks.
Undiagnosed/ un treated Hypothyroidism is a condition that can affect milk production but isn't hugely common in reality. It can be developed a little while after birth too. It has to be pretty bad though I think - my tsh was around 13 at one point and I still had quite a lot of milk.