Here's the behavior concepts:

You can use 'dispensers' to dispense any type of 'minecart' onto 'rails' in any orientation, so long as the 'dispenser' is facing the rail from any of the 6 directions, or if the dispenser is facing the space above the 'rail' from the 4 cardinals.

When any 'minecart' is placed on an unpowered 'powered rail', it will not move, regardless of orientation, or how it was placed. 

When two 'rails' are placed next to each other, if one 'rail' is a block higher, the lower of the two will slant or climb up to the higher 'rail'. This slant persists even if the higher 'rail' is moved or destroyed, until the slanted 'rail' is connected to another 'rail' from any other side.

When a 'minecart with hopper' is placed onto a 'rail', it will be able to extract items from containers above it, unless it is placed on a powered 'activator rail' or until it crosses one. 

But here's the bug:

When you dispense a 'minecart with hopper' onto a slanted, unpowered 'powered rail' from the direction the 'rail' is climbing to, the 'minecart with hopper' does not extract items from a container above it.

This scenario works normally if the 'minecart with hopper' is placed on the slanted, unpowered 'powered rail' by the player or dispensed from any other valid position.  But it doesn't work from that one position.

I've tested this in every cardinal direction, with every container, dispensed from all valid positions of a 'dispenser', with a variety of items, 64x stackable, 16x stackable and non stackable, on 1.15 prerelease 5 and 6, and this bug persists. 

Additionally, when dispensed from the side the 'rail' is climbing from, the hitbox of the 'minecart' is identical vertically to that of a cart dispensed from the side is climbing to, yet the hitbox horizontally is biased away from the 'dispenser' it came from, and the 'minecarts' visually appear to be at different heights on the 'rail'. Though this may be intended, I included this as another related bug, just in case.