Entities/mobs who are given the resistance buff via commands/command blocks lose it when the server goes offline. The resistance buff is still there after the server goes online again, however it is not actively working, and the inactive buff will be treated by the game as if it is still there(eg. when using command blocks to detect for resistance in order to reapply resistance using if|unless, it will not apply a new resistance buff because that inactive one is there). This is exclusive to the resistance buff, none of the other buffs such as strength and speed have this issue.