Realms specific, not tested in Single Player
What I expect to happen
When placing a redstone block under a continuous line of powered rails I expect 8 rails on either side of that active powered rail to be active. 

What is happening
While it is typically 8 I am having issues where the number powered fluctuates between 8 and 12 blocks seemingly randomly. Breaking and replacing a rail fixes this. However on reload of the chunk the randomness seems to still take effect.

Impact
I want to have one unpowered rail at the end of the minecart line that is powered by a button to 'launch' the minecarts (and to prevent a bump from sending them moving accidentally), but on reload of the chunk the rails are sometimes powered and sometimes unpowered. I cannot give more then two-three blocks of 'leeway' because the button doesn't power the rails long enough to clear them (especially for my console players). 

Add-Ons
I do have several add-ons installed, however none of them have anything to do with redstone and I ensured all blocks involved in the rail are non-add-on blocks. The rails were created with gold from Feed the Bees, but I cannot see how that in any way would affect the behavior. I only mention it for completeness.