I am having this exact same problem. Also using the 4-2-3-1 tactic. I've recently started switching to a 4-2-3-1 with defensive midfielders when trying to protect a lead, but so far the results have not been good.
Im in my second season at the premier league and last season we only kept a clean sheet ONCE after scoring. It's getting very frustrating as it seems the problems are still there, as I'm now 3 games in to my second season and I've drawn one game and lost two and in every game I have taken the lead at some point.
I cannot figure what the problem is since I also use a completely different 4-1-2-1-2 tactic and it doesn't help.
I don't think changing the formation would do much to stop this (unless you change to something ridiculously defensive like having six at the back), the tactics themselves are more important. It's possible that your tactics are vulnerable to particular styles of play that are likely to happen when you've just scored. After a goal I'd expect the conceding team to raise their tempo and play more direct football, and if that's your weakness then that could be the problem.
It might not be just the tactics though, it's also possible that players having low concentration or determination comes into play in this situation. It's all about blending the squad, the formation and the tactics together so that it all works perfectly, so only knowing what formation you use doesn't really help when trying to find out the problem.
I guess I didn't clearly state that my other tactics are more defensive when it comes to player roles, passing etc.
The 4-2-3-1 I normally use is what I usually start with.
It contains an advanced forward, two supporting wingers, a deep lying playermake(S), an APM(A) and a CM(automatic), full back(S), CD(stopper), CD(cover) full back(S).
Where as the more defensive 4-2-3-1 has: AF, APM(S), 2 support wingers, 2 defensive midfielders, 2 limited defenders and 2 full backs(D)
I use the latter one for the later stages of the game, although it doesn't seem to help so I might have to re-think it.
Also the 4-1-2-1-2 I use has the same problem of conceding after scoring. Average determination and concentration in the squad are both around 12-15 depending on which players I include. Despite conceding often we also score quite often, resulting in very big number games. For instance we just beat Man city 5-4.
The problem is that there are so many different things that could be causing this problem for people, makes it impossible to guess. Off the top of my head, stuff that could cause a team to do this are the things I've already mentioned (concentration, determination, formation), form, player relationships, tactic familiarity, team mentality, player personalities.
Your roles and formations sound fine, and the stats don't seem too bad. My team has stats from 9 - 19 for concentration (with the highest being defenders) and 15-19 determination, so while there's a small difference I wouldn't expect it to have that big an effect.
I think a major thing that makes my team successful is that I have a load of different tactics (using my tactics system, article here if you wanna find out more!), and that I select the team and alter the tactics to suit the opposition whenever I come up against anyone above average. It also means my team has very good familiarity with the tactics, so they're in a system they're used to.
A bit more info about what I've done in my save just in case it'd help you:
I've had the same squad at Man Utd for a couple of years now with the training focus set to team cohesion (no new players bought, several sold/retired were replaced with some really good regens that I've been lucky enough to get). The game has become ridiculously easy, to the point that I'm really getting bored with the save.
At the start of Feb 2016, I've played 22 games in the league (won 20, drawn 1, lost 1), with 94 scored 15 conceded. I won every champions league group game, with 25 scored 7 conceded, and am still in every cup competition. I've already won the Community Shield, Euro Super Cup and the Club World Championship this season, and City in second are 12 points behind having played an extra game in the league. Standout scores include 11-2 vs Arsenal, 10-0 vs Wigan, 4-1 vs Man City, 5-0 vs Liverpool, 10-0 vs Sydney FC, 6-0 vs Sao Paulo, 7-0 vs Aston Villa, 4-0 vs Chelsea.
Like I said, there's so many different things that could be causing this to happen that it's almost impossible to find out what's wrong. The easiest thing to do is to look at the way you play the game, particularly how you prepare your team for matches, and try to improve as much as you can.