What is the ruling on a sexual relationship that did not go beyond the hymen between an engaged couple after the Fatiha (engagement ceremony) and before marriage, and is it obligatory to inform the current fiancé about it?

If the marriage contract (kitab al-kitab) was a complete, sharia-compliant marriage contract fulfilling all conditions and pillars, then he is your husband, and whatever is permissible for a husband from his wife is permissible for him from you. However, if it was merely an engagement (khitbah), then nothing is permissible for him from you except the sharia-sanctioned glance (al-nadhra al-shar'iyyah). Anything beyond that, such as looking, seclusion (khalwah), or fondling, is forbidden. If anything forbidden occurred between you during the engagement period, you both must sincerely repent and not return to it. The expiation for sin is sincere repentance and charity. You must conceal what happened and not inform a second suitor about it.