Descriptive Problem Summary:
When using a plane master and a render target alpha mask, the plane master and objects having different blend modes it will move the mask parallel with the player, rather than remaining still
Numbered Steps to Reproduce Problem:
1) Run test case
2) Move in any direction

Code Snippet (if applicable) to Reproduce Problem:
Test case: https://github.com/TiviPlus/alpha_bug_repro

Expected Results:
It creates an alpha mask on the target that does not change when the player moves, which masks the object

Actual Results:
The mask will seem to act like a parallax limited to that specific object
In a game: https://streamable.com/2y8p1a
Alternate test case: https://streamable.com/vczz1p
Final Test case: https://streamable.com/oet2nk

Edit: https://streamable.com/mt7mus here you can better see where it is supposed to be with the white(how it is supposed to be, unmoving), and the more transparent (one which keeps moving and this bug is about)

Does the problem occur:
Every time? Or how often? Yes
In other games? Yes
In other user accounts? Yes
On other computers? Yes

When does the problem NOT occur?
Never

Did the problem NOT occur in any earlier versions? If so, what was the last version that worked? (Visit http://www.byond.com/download/build to download old versions for testing.)
Reproduced on latest stable(513.1542) and several betas including latest(514.1561)
Workarounds: