I don’t think these are the same thing. I think “anal creampie” implies current penetration, while “semen_in_anus” could just be an after_sex kind of thing. I could be wrong, though.
Edit: Well, the usage seems to contradict me… https://chan.sankakucomplex.com/?tags=anal_creampie%20-anal&page=1
I’ve submitted it.

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jHAS
1 month agoTag Alias: anal_creampie -> semen_in_anus
Aliasing anal_creampie -> semen_in_anus.
Reason: same thing