We are having a problem when it comes to the caching loading within portal. Basically we have two image services, one tiles correctly (we'll call this one Cache 1) and one waits for every tile to load then displays all at once (Cache 2). When using Cache 2, even when you are zoomed in to an area, it still loads all tiles, even those not currently displayed on the screen.
When we created the newer cache (Cache 2), I copied all the same settings as the first Cache 1. Although there are some slight differences, I don't think they are enough to cause the issues we are having. Once difference I made was I made the lowest level at 1:250, whereas the first one has the lowest level at 1:1128. The only other difference between the two is that Cache 1 was created in 2018, using ArcMap 10.1 and Cache 2 was created in 2020 using ArcMap 10.4.
Another weirdness with the loading is when you view the imagery from the Server Manager online or the REST Server, both caches tile correctly. It is only when they are run through the ArcPortal API that the cache loading issues are apparent.
We are currently running Portal Version 10.5 and Server Version 10.6