I get the same issue in version 11.3 (Wolfram Cloud):

    $Version
>"11.3.0 for Linux x86 (64-bit) (March 7, 2018)" 

    bl = BarLegend["Rainbow", LegendMarkerSize -> {100, 300}]
[![enter image description here][1]][1]


This is caused by the option `FrameTicks -> {{False, False}, {True, False}}` (that gets in there somewhere along the way):

    Cases[ToBoxes[bl], p : Rule[FrameTicks, _], Infinity] 
> {FrameTicks -> {{False, False}, {True, False}}} 

A work-around: post-process the box expression to modify `FrameTicks` option value:

    RawBoxes @ Replace[ToBoxes[bl], Rule[FrameTicks, _] :> Rule[FrameTicks, False], ∞]
[![enter image description here][2]][2]

Alternatively, change the setting for `FrameTicksStyle` to `White`:

    RawBoxes@Replace[ToBoxes[bl], Rule[FrameTicksStyle, _]:> Rule[FrameTicksStyle, White], ∞]

> same picture


  [1]: https://i.stack.imgur.com/7CEEy.png
  [2]: https://i.stack.imgur.com/7xqwo.png