In BC, Canada, if a tenancy agreement is signed by a brokerage/property manager on behalf the landlord, and the property manager's contract is terminated early, will that make the tenancy agreement invalid? Does the tenant need to re-sign with the landlord directly? Or does nothing need to be done?
on behalf means that the party of the agreement is the landlord, not the property manager. The contract both entitles and obliges the landlord, not the property manager. The property manager is not a party of the contract.
So the fact that the property manager is fired completely unrelated to the existing contract.
Additionally, in most jurisdictions that I know of, even if the property changed ownership (the landlord sold or gifted it, or the landlord died and it was inherited by someone) the contract would still be in force, as the change of situations would not invalidated the rights and obligations of the other parties.