As a workaround
$Version
"12.1.1 for Mac OS X x86 (64-bit) (June 19, 2020)"
data = {{{1500, 4}, {1510, 4}, {1520, 4}, {1850, 6}}, {{1500, 1}, {18100, 1}}};
labels = {"a", "b"};
Show[
ListPlot[#[[1]],
PlotStyle -> ColorData[97][#[[2]]],
PlotLabels -> #[[3]],
Joined -> True] & /@
Transpose[{data, Range[Length[data]], labels}],
PlotRange -> All,
AxesOrigin -> {0, 0}]
Alternatively,
Show[
ListPlot[#1,
PlotStyle -> ColorData[97][#2],
PlotLabels -> #3,
Joined -> True] & @@@
Transpose[{data, Range[Length[data]], labels}],
PlotRange -> All,
AxesOrigin -> {0, 0}]