Symbola.ttf font problem on OSX High Sierra

For some time now I have been running the following GNUplot script to generate 3 day tide charts with a moon phase placed at the time it occurs.

Nothing in the setup and plotting script has changed since a recent OSX upgrade from Yosemite to High Sierra.

The moon phase icons are generated by using the Symbola.ttf font and all worked well on OSX Yosemite but now fail on the First and Last Quarter plots, but not on the Full and New Moon plots.

Symbola.ttf is found in all locations.

echo $GDFONTPATH
/User/library/Fonts:/System/Library/Fonts:

gnuplot> show fontpath
fontpath is 
    system fontpath is "/System/Library/Fonts!" "/Library/Fonts!" "/Users/house/Library/Fonts!" 

I am not including the various setup parameters here but will do so if needed.

Here is a modified data file showing the four moon phases (I've compressed it so as not put up a full month of data here). threedaytidaldataOnlyfile looks like this but with a full three days data:

2019-01-05 10:28   9.84 feet  High Tide
2019-01-05 16:54  -0.37 feet  Low Tide
2019-01-05 20:29   New Moon
2019-01-06 11:09   9.77 feet  High Tide
2019-01-06 17:33  -0.29 feet  Low Tide
(Snip)
2019-01-13 15:53   8.19 feet  High Tide
2019-01-13 22:04   0.93 feet  Low Tide
2019-01-14 01:46   First Quarter
2019-01-14 04:26   8.40 feet  High Tide
2019-01-14 10:35   1.29 feet  Low Tide
(Snip)
2019-01-20 22:27   9.50 feet  High Tide
2019-01-21 00:17   Full Moon
2019-01-21 04:20  -0.58 feet  Low Tide
2019-01-21 10:38  11.25 feet  High Tide
(Snip)
2019-01-27 03:48   9.78 feet  High Tide
2019-01-27 09:59  -0.13 feet  Low Tide
2019-01-27 16:12   Last Quarter
2019-01-27 16:16   9.24 feet  High Tide
2019-01-27 22:24  -0.04 feet  Low Tide

Here are the plot commands:

set terminal pngcairo size 1000,400 (pngcairo 'enhanced' has no effect)
plot threedaytidaldataOnlyfile \
u (myDate(1,2)):3 lc rgb "green" lw 1  sm cspl notitle,\
"" u (myDate(1,2)):3:((strcol(5) eq "High") ? (myLabel(3,2)) :1/0) with labels font "Gill Sans Light,12" offset 0,0  tc rgb "blue",\
"" u (myDate(1,2)):3:((strcol(5) eq "Low") ? (myLabel(3,2)) :1/0) with labels font "Gill Sans Light,12" offset 0,1  tc rgb "red",\
"" u (myDate(1,2)):(strcol(3) eq "Full" ? yAverage : $3):((strcol(3) eq "Full") ? ("◯"):1/0) w labels left font "Symbola,28" offset -4,0,\
"" u (myDate(1,2)):(strcol(3) eq "Full" ? yAverage : $3):((strcol(3) eq "Full") ? (moonPhaseTimeName(2,3)):1/0) w labels left font "Gill Sans,14" ,\
"" u (myDate(1,2)):(strcol(3) eq "First" ? yAverage : $3):((strcol(3) eq "First") ? ("☽"):1/0) w labels left font "Symbola,28" offset -4,0,\
"" u (myDate(1,2)):(strcol(3) eq "First" ? yAverage : $3):((strcol(3) eq "First") ? (moonPhaseTimeName(2,3)):1/0) w labels left font "Gill Sans,14",\
"" u (myDate(1,2)):(strcol(3) eq "New" ? yAverage : $3):((strcol(3) eq "New") ? ("●"):1/0) w labels left font "Symbola,28" offset -4,0,\
"" u (myDate(1,2)):(strcol(3) eq "New" ? yAverage : $3):((strcol(3) eq "New") ? (moonPhaseTimeName(2,3)):1/0) w labels left font "Gill Sans,12" ,\
"" u (myDate(1,2)):(strcol(3) eq "Last" ? yAverage : $3):((strcol(3) eq "Last") ? ("☾"):1/0) w labels left font "Symbola,28" offset -4,0,\
"" u (myDate(1,2)):(strcol(3) eq "Last" ? yAverage : $3):((strcol(3) eq "Last") ? (moonPhaseTimeName(2,3)):1/0) w labels left font "Gill Sans,14"

Here's what the proper output .png file should be.

@http://suiattle.net/transfer/threedaystides.png

Here's what happens only with the First and Last Quarter

@http://suiattle.net/transfer/threedaystides-bad.png

Can anyone suggest why the First/Last Quarter lines fail?