Not a bug
BYOND Version:513.1497
Operating System:Windows 10 Pro 64-bit
Web Browser:Firefox 68.0
Applies to:Dream Seeker
Status: Not a bug

This is not a bug. It may be an incorrect use of syntax or a limitation in the software. For further discussion on the matter, please consult the BYOND forums.
Descriptive Problem Summary: If another turf is in vis_contents, but on a different plane, it ignores KEEP_TOGETHER and alpha masking.

Numbered Steps to Reproduce Problem:
1. Have turf on plane
2. Have object on higher plane
3. Put turf into object's vis_contents. Ensure object has KEEP_TOGETHER.
4. Turf ignores KEEP_TOGETHER and alpha masking

Code Snippet (if applicable) to Reproduce Problem:
plane = 2

icon = 'stuff.dmi'
icon_state = "portal"

vis_contents += locate(/turf/b) in world
filters += filter(type="alpha", icon = icon('stuff.dmi', "portal_mask"))

Expected Results:
Turf in vis_contents stays within the alpha mask

Actual Results:
Turf in vis_contents ignores alpha mask

Does the problem occur:
Every time? Or how often? Yes
In other games? Reproduced in a demo

When does the problem NOT occur?
When planes are unmodified and left to their defaults

Don't edit planes
Lummox JR resolved issue (Not a bug)
Items on other planes automatically use KEEP_APART. That's expected behavior.

Using VIS_INHERIT_PLANE in vis_flags would force it onto the same plane as the container, though, and therefore avoid the keep-apart.