Fixing bug #54243 would make this bug irrelevant, but if that is not done for some reason, then here's the inconsistency:
In most cases, when a new sink input is created, and the sink input needs to be made audible immediately, the sink implementor requests a rewind in the sink input state change callback when the state changes from PA_SINK_INPUT_INIT to PA_SINK_INPUT_RUNNING. module-combine-sink could do the same, but instead it requests the rewind in the attach callback. That would actually be a potentially user-visible bug if the sink inputs that module-combine-sink could be moved, but since the inputs can't be moved, requesting the rewind in the attach callback is equivalent to doing it in the state change callback. There's no reason to do it differently in module-combine-sink than in other modules, so for consistency module-combine-sink should be changed to request the rewind in the state change callback like everyone else.