Template not rendering and mainlabel=- not observed

[vid]

Setup

• MW version: 1.38.2
• PHP version: 7.4.30
• DB (MySQL etc.): MariaDB
• SMW version: 4.0.2
• MT version: 1.2
• Browsers and versions (if applicable):
• Chrome

Issue

The template is not rendering variables, it just shows the variable names. And it shows mainlabel, even though that is by convention not the right option.

Here is the query (I have tried variations):

{{ #ask: [[Probable year::+]]|[[Short title::+]]
|mainlabel=-
|?Short title
|?Probable year
|?Page name
|?Right
|?Section heading
|format=moderntimeline
 |start at end=on
 |height=400px
|scalefactor=39
|limit=5000
|navigation height=80%
}}

Here is the template:

[[{{{3}}}|{{{6|{{{7}}}}}}]]

{{{4}}} {{{5}}}

Steps to reproduce

Use data with the above query and template.

Expected result:

The template should render with values, and the timeline items should use the Short title, not the mainlabel.

Observed result: ...

Not the right thing.

Thanks!