Please note that Realms support is unstable. Mojang is working on updating Realms to better support Marketplace content. We do not have control over Realms issues. Please contact Mojang to report any issues you have with Marketplace content - especially HD texture packs.
There was a bug that caused corrupted, partial downloads of HD texture packs. This has been reported to the Realms team and a fix should be out soon if not already.