Hi,
Firstly, we want to apologize for all the inconvenience caused in this regard.
Secondly, we have checked and further probed this concern on your Site and found out that by disabling the Custom Layout for the Single Product page under the Dashboard> Layouts > the Thumbnail Images were appearing as you can see in the following Image: https://ibb.co/JK2gL4M
So, it seems that there is some third party Plugin element or some third party Custom Code added in your Custom Layout which is causing the Thumbnails to not work correctly.
So, if you have used any of the above mentioned in your Custom Layout then kindly remove that and then enable the Layout again. Hopefully, you won’t find the issue again in this regard !
Best Regards.