since before… 在英语中很常见（意思是“自……前就一直”），如 since before the war, since before Christmas 等；但 since after… 似乎没怎么见到，要表示“自以……以后”，直接用 since 就可以了，比如 since the war, since Christmas（一般不会说 since after the war, since after Christmas）。

或许你问的是同样是介词since加pp即before...为啥比since加pp即after...要常见很多？这是一个很值得讨论的问题。我的看法是：你列出的2句都是对的，不同的语境下。

剑桥语法没有这个问题的答案，但是在P638页有这样一句：

They have lived here [since just after the war].

Declerck的著作“The Grammar of the English Verb Phrase Volume 1 The Grammar of the English Tense System A Comprehensive Analysis”的第607页有这段话：

The combination of since with after is rather unusual, but not impossible:

The French ministry emphasized that France has not authorized the sale of weapons, or even spare parts, to Iraq since after July 1990. (www)

After being freed, he immigrated to America and has lived in the US since after

the war. (www)

Although Western films have been lensed in Vietnam since after the war, some

have been turned away after an initial OK. (www)

有人说“since after the war是一个已经词汇化、习语化的特殊搭配，也很正常”。这么说不无道理，但通过语料分析发现，搭配多种多样，远不止只有这一种形式。

我觉得吧，since before X 的高频，是因为它特别适合表达早在 X 以前已经成立并跨越 X 持续的信息；since after X 的低频则不是语法限制，而是它只有恰好需要以 X 为参照（X是个分水岭）、而又只想说明真正起点晚于 X时，since after X 才成为合适的形式。

只列举一部分当代的例子，AI作为强大的工具，不好好利用实在可惜！