De-Jure / Legally
This question asks about the "de jure" and "legal" situation.
Whenever you ask a legal question, you have to specify the jurisdiction that applies. In this case, the only relevant jurisdictions seem to be one of
- International law
- UK law
- Israeli law
- Palestinian law
International law is not really law in the normal sense. As this answer to a different question explains, it is a set of somewhat nebulous conventions that most national governments mostly choose to follow. There is no supra-national law court that applies International law to nations or which prosecutes offending nations. The ICC prosecutes individuals, not nations.
The UN is not a world government, it relies on its members to enact national laws and it relies on its members to enforce those laws.
The ICJ can adjudicate on disputes between member states but, for example, the USA withdrew from compulsory jurisdiction in 1986 and only accepts ICJ verdicts that suit it. "The key principle is that the Court only has jurisdiction on the basis of consent. The court has no true compulsory jurisdiction."
It seems unlikely that Israel would want to dispute the termination of the mandate and ask the ICJ to rule that the UK was the legal civil power in all of mandatory Palestine. It seems unlikely that the UK would consent.
It seems unlikely that any court in the UK, Israel or the current Palestinian territories would proclaim that the UK was the current legal civil authority in what was formerly mandatory Palestine and is now Israel and the Palestinian territories. So far as I know, the question has not been put to a court - at least not to a court that is recognised by all the affected countries as having legal jurisdiction over the matter. You cannot know the legal situation for certain if a court with jurisdiction has not ruled on the matter.
The mandate was an agreement between an organisation and one of its members. The member gave notice of termination of the agreement and the organisation, though very active since, in resolution 181 took note of the mandatory power's withdrawal and has notably not at any time in the last seventy years asserted that the agreement is still in effect.
My conclusion is that, if all the above is correct and that none of the involved parties contest termination, it makes little sense to ask whether the mandate legally ended.