[API] Correctly expose slur anchor points

[XiaoMigros]

Resolves: #31159

Regression, works correctly in 4.5x

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• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
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• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)

[cbjeukendrup]

@XiaoMigros does this need any testing from our side, or are you confident you have tested it sufficiently yourself?