Is ghusl obligatory after feeling something ejected during wudu and mandatory prayer, with a semen stain present, despite not recalling a wet dream or seeing a dream?

If you have ascertained that what was discharged from you is manī (seminal fluid) with its known characteristics, and it was discharged with a gush and pleasure, then ghusl (ritual bath) is obligatory upon you by the consensus of the scholars, based on the Prophet's (peace be upon him) saying to Ali: "If you see madhī (pre-seminal fluid), wash your penis and perform ablution (wudu') for prayer. But if you gush forth water (manī), then perform ghusl."

However, if it was discharged without pleasure, it does not necessitate ghusl according to the majority of scholars, in contrast to the Shafi'is who hold that the discharge of manī necessitates ghusl, whether with or without pleasure.

If it does not possess the characteristics of manī, then it is most likely madhī, which does not necessitate ghusl. For it, wudu' is sufficient, along with washing whatever of it touched the body and clothes. Your prayer is valid unless you are certain that its discharge occurred before or during the prayer.