text-allow-overlap is ignored when symbol-placement= line-center

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When I set the following for symbol layer, the label still disappears when text collide or when zoomed out.
```
{
"symbol-placement": "line-center",

"text-allow-overlap": true
}
```

**mapbox-gl-js version**: v1.10.1

**browser**: Chrome 83

### Steps to Trigger Behavior

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### Link to Demonstration

https://codepen.io/tallema/pen/KKdLpGZ?editors=0010

### Expected Behavior

The text will show at each zoom level

### Actual Behavior

Text disappears at for lower zoom levels

Start with the linked CodePen reproduction and compare the symbol layer behavior at multiple zoom levels with symbol-placement set to line-center and text-allow-overlap enabled. Trace the label placement and collision behavior for this configuration, then verify that the text remains visible at each zoom level without regressing other symbol-placement modes.

Written by the indexing model from the issue text.