Default initiation selection bar width not correct

[seehui]

In bootstrap modal box (mobile size), when first time load the range slider, the selection bar width is about 1/3 of the bar. Max and min value is correct.

But after dragging the slider, the bar gets back to normal, with max value expanded to full bar width.

Here are the initial loading bar width:

Here are after dragging bar width, which should load initially:

Anyone know how to fix this? Thanks in advance.