Spurious '38h's appear at all vertices on Matrix Brandy BASIC Tek output

This may be more of an issue with stardot/MatrixBrandy BASIC's Tek output thank Tek4010's rendering, but testing output with three different Tek rendering devices gives the following results:

xterm: output as expected
tek2plot (GNU plotutils): output as expected
Tek4010: spurious output on display.

The following Tek commands from Matrix Brandy BASIC should draw a square rotated 22° anticlockwise in the centre of the screen:

00000000: 1b5b 3131 3b33 306d 1b5b 3430 6d1b 5b3f  .[11;30m.[40m.[?
00000010: 3338 681b 0c1b 031b 5b32 4a1b 5b48 1b5b  38h.....[2J.[H.[
00000020: 3f33 3868 1b03 1b5b 3f33 3868 1d2f 7128  ?38h...[?38h./q(
00000030: 4234 6333 4b1f 1b03 1b5b 3f33 3868 1d2f  B4c3K....[?38h./
00000040: 7128 4224 682c 541f 1b03 1b5b 3f33 3868  q(B$h,T....[?38h
00000050: 1d24 682c 5428 7a37 5d1f 1b03 1b5b 3f33  .$h,T(z7]....[?3
00000060: 3868 1d34 6333 4b28 7a37 5d1f 1b03       8h.4c3K(z7]...

(zipped output: http://scruss.com/wordpress/wp-content/uploads/2019/11/mtxbrandy_box_tek.zip)

This is what the output from tek2plot looks like: http://scruss.com/wordpress/wp-content/uploads/2019/11/mtxbrandy_box_tek2plot.png

The output in an xterm window looks similar, and is as expected.

The output from Tek4010, however, has spurious characters at the top left and at every vertex: http://scruss.com/wordpress/wp-content/uploads/2019/11/mtxbrandy_box_tek4010.png

rricharz / Tek4010

Spurious '38h's appear at all vertices on Matrix Brandy BASIC Tek output #17