A WHO document, contains the statement
[Melarsoprol] injections must be performed using glass syringes.
Why is this the case?
Since Melarsoprol is insoluble in water, dosage occurs via a 3.6% propylene glycol intravenous injection.
As propylene glycol can dissolve plastic, the drug should preferably be administered using a glass syringe (only if sterilisation is reliable), otherwise inject immediately (but slowly) using a plastic syringe (Source).
Melarsoprol is sometimes colloquially referred to as "arsenic in antifreeze".
The reason for it may not be the barrel of the syringe, but the rubber in the stopper which is of a different material. There was a study done on paraldehyde and the affect it had on the plastic in syringes and needle hubs. The pertinent part (Note the time, there was no effect until after at least 3 hours immersion) is below (Emphasis mine).
No measurable change in residue weight was noted in any syringes for up to three hours. Compared with the control, there was a significant increase in the average weight of residue in the Glaspak and in the Plastipak syringes at 6, 12, and 24 hours. There was no significant difference in the weight of residue between the Glaspak and Plastipak syringes at those times, however. The amount of residue for the plastic and metal needle hubs was not significantly different. The source of the extractive residue appeared to be the rubber plunger tip. Since the nature of the extractive material in the residue is not known, paraldehyde should be administered in all-glass syringes if possible; other syringe types can be used only if the drug is administered immediately.
So basically, it's possibly erring on the side of caution as they don't know how soon it might start dissolving stuff.