Note: Whether a glitch is normal or indicates a problem depends on many factors best evaluated by a professional, such as a geriatric psychiatrist or neuropsychologist. It's important to realize that changes in cognition and memory tend to fall along a spectrum. This symptom is considered a sign of concern that warrants an evaluation if it happens consistently or begins to interfere with daily life, especially if this is a change (new or different).
Why it happens
Planning involves several kinds of higher-order thinking, from weighing possible options and sequencing prospective events to thinking abstractly. These cognitive functions can be affected early.
What you can do
Have patience and allow extra time.
Encourage collaborative thinking where possible, so that you can guide the planning process along and point out the necessary considerations and steps that must be taken to implement a plan.
Don't expect the person you're concerned about to continue former responsibilities without help (such as financial planning, vacation planning) if you expect the same level of results; gently step in to do it together or divide tasks.
Be mindful of work life being affected by this tendency. Do consult a physician if work or daily life becomes negatively affected.