I also went from being 10 points clear in my league to losing 6 in a row with no explanation, even against lesser teams. In the end i took drastic action and sold my goalkeeper (too many goals conceeded, could be knocking on to the confidence of defenders) and bought one from the list with one on one skill. I also ditched 4-5-1V formation as that has stopped being so effective. There's another thread about 451v now not working against 4-4-2 which makes me think there's been a change in the programming code.
Previously 451 formation had worked for me then i noticed it was seemingly leaving me very open to counter attacks (even though majority of time i only selected own half pressing. So i switched it up. Now won 4 in 5 but 3rd in my league and two very tight games. If i'm lucky i'll end up 2nd but my team went from winning or drawing to losing 6-0 to a team one below me, 3-0 to unmanaged sides and it was really frustrating. Would suggest some big changes to get season back on course.