I submitted the following to the Debian bug tracker #931305.

While using proportional fonts, the Greek letter pi (π) is treated as a box-drawing character or, more likely, as missing from the proportional font altogether. This happens only at certain point sizes AND/OR only with specific fonts.

Scenario 1: Greek pi works, but box-drawing does not

With these settings:

xterm.vt100.faceName: DejaVu Sans Mono
xterm.vt100.faceSize: 10
XTerm.vt100.forceBoxChars: false

The greek letter pi is displayed correctly, but the second vertical line (drawn with U+2502) is almost the same as the first one (drawn with U+007C).

Scenario 2: Greek pi does not work, but box-drawing does

With these settings:

xterm.vt100.faceName: DejaVu Sans Mono
xterm.vt100.faceSize: 10
XTerm.vt100.forceBoxChars: true

The Greek letter pi is drawn using a fixed-size (bitmap) font. The second vertical line is properly displayed using box-drawing characters.

Scenario 3: faceSize: 9.5 forceBoxChars: false works for both

With these settings:

xterm.vt100.faceName: DejaVu Sans Mono
xterm.vt100.faceSize: 9.5
XTerm.vt100.forceBoxChars: false

Everything appears to work as intended.

Scenario 4: Fira Code works using settings from scenarios 1 and 3

With these:

xterm.vt100.faceName: Fira Code
xterm.vt100.faceSize: 10
XTerm.vt100.forceBoxChars: false

Or this changed:

xterm.vt100.faceSize: 9.5

Everything seems to work as intended.

Scenario 5: forceBoxChars always breaks Greek letter pi (π)

With these settings:

xterm.vt100.faceName: Fira Code
xterm.vt100.faceSize: 10
XTerm.vt100.forceBoxChars: true

Regardless of typeface, enabling forceBoxChars will always draw the letter pi in a bitmap font.