Reference quality objectives, is it ok to reduce the original target value if there is difficulty in meeting the objective and then investigate why it cannot be met? Will this create an issue for ISO 9001 external audits?
No, it will not be a reason for nonconformity during an external audit, unless it is something that the company does systematically. For example, in a year like 2020 with events like the coronavirus, it is natural that many objectives, depending on the context, are not attainable.
The following material will provide you more information:
Free webinar on demand - Measurement, analysis, and improvement according to ISO 9001:2015 - https://advisera.com/9001academy/webinar/measurement-analysis-and-improvement-according-to-iso-9001-2015-free-webinar/