When joining a realm that has the same resource pack as the one, you're currently using if yours is set on the client side to any resolution other than the one on the furthest right resolution on the slider it will change to the one on the furthest right. 

Steps to reproduce:
Step 1: Upload resource pack to realm Ex. PureBDcraft (Require players to accept resource packs to join should be toggled off)
Step 2: Load same resource pack on the client side with a resolution set to any option that isn't the furthest right one on the slider. 
Step 3: Join Realm you uploaded the resource pack to.

Observed Result:
When you load into the Realm your Resource pack will load at the furthest right resolution on the slider.

Expected Result:
When joining realm resource pack loads at clients preferred resolution.

Tested using PureBDcraft resource pack. Currently the resolution slider for this pack is [32x 256x 128x 64x] with any option chosen changing to 64x when joining a realm with the same resource pack. Previously the slider for the pack was [32x 64x 128x 256x] so when you joined a realm it defaulted to 256x which was the furthest right option.

Edited for clarity and updated with new information.