This is because Ubuntu Mono does not contain Arabic support. The Arabic glyphs are being substituted, from another (proportionally-spaced) font. The solution means adding Arabic Glyphs to Ubuntu Mono; or in the short term, trying to adjust the fallback sequence so that fallback is done to another monospace font (rather than to a proportional font).
This is because Ubuntu Mono does not contain Arabic support. The Arabic glyphs are being substituted, from another (proportionally -spaced) font. The solution means adding Arabic Glyphs to Ubuntu Mono; or in the short term, trying to adjust the fallback sequence so that fallback is done to another monospace font (rather than to a proportional font).