The same thing happened to me in 2021. Our shore excursion was cancelled on our cruise on 8 September 2021. The refund was shown on our onboard account, and we were told it would be refunded to our credit card (used to book the excursion) within 10 working days. After getting nothing for 30 days we did everything requested by NCL - filed a claim, got a claim number etc.
After several emails (to the named “Guest Experience Co-ordination”) we eventually got our money refunded in March 2022 - so 6 months!
Luckily they owed us $173, rather than hundreds of dollars, but still very annoying and time consuming to keep following up.
So I’m afraid I don’t have any advice really, but I just wanted to reassure you that we did eventually get our money back via our registered credit card.