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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

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], Infinity]

enter image description here