This is most likely a workflow/software issue, not a printer-specific fault. Based on the reported fix, Lightroom needs to remain open until the entire print job has fully finished spooling and printing. If you close Lightroom while pages are still being printed, the remaining pages may print with darker color.

So the practical solution is: do not quit Lightroom until the printer is completely done.

If the problem still happens even with Lightroom left open, a secondary possibility mentioned was an ink-related issue, so checking or replacing the full ink set could be worth trying. But since the same behavior occurred on different printers and computers, keeping Lightroom open through the end of the job is the strongest answer here.