Defence Disappears

My team has some amazing strikers and we can go 3-0 up by the half, only to lose 3-5 at the end. My team has thrown away a 4-0 to draw 4-4.
It drives me crazy. My tactics have got me to the top of the Prem and to a Champions League win, but the rest of the time it's a goal-fest against me regardless of my lead.
Why?

Do you make any tactical adjustments during the match?

The AI will make changes when their team is not playing well, and if you keep the same formation throughout the match, you can be exposed.