It sounds to me like bad practice, but it may not be unlawful. Decision making can be delegated to the clerk, and the clerk also has a duty to avoid the council falling into unlawful or damaging situations. It seems to follow from this that, if the clerk realised that it was not going to be possible to publish proper notice of the meeting, or to hold it with a quorum, at the originally agreed time, then it would be right for the clerk to fix an alternative date so as to avoid failing to hold the annual meeting within the prescribed period. That would not be a decision by the council, even if the clerk had spoken to councillors, it would be a decision by the clerk. As you're doubtless aware, nothing can be decided by the council outside a properly constituted formal meeting, and all decisions must be minuted. The clerk should report any decisions they have made to the council at the first opportunity.
However, this sort of thing should not need to happen. The council is entitled to ask the clerk to give an explanation, and if not satisfied, to say so.