Did you set up that printer specifically as an AirPrint printer? If not, please do that.
Presumably, the hour you spent on the phone with Epson referred you to the following passage from The option for 2-Sided Printing (Windows) or Two-Sided Printing Settings (Mac) is gray or not available. What should I do?
Make sure you use a paper type and size that supports this feature. Automatic double-sided printing is only available if you select plain paper and these paper sizes in the printer software:
- Letter (8.5 x 11 inches [216 x 279 mm])
- Legal (8.5 x 14 inches [216 x 357 mm])
- A4 (8.3 x 11.7 inches [210 x 297 mm])
Note: Automatic double-sided printing is not supported on glossy, thick, and certain matte papers.
If that doesn't help, one can only assume it's due to some unexplained interaction between that printer, its driver, macOS, or all three.
Since Epson places the blame on Apple, you could try contacting them, but their own Support document already says to contact the printer's manufacturer, leaving them pointing fingers at each other, and you out in the cold.