Confounding port failure on 2016 MBP

On our 2016, 13" MBP, a peculiar port failure issue.

When we attach an external USB-C drive to the FORWARD, right port—the system fails to mount it. The drive is getting power, as indicated by the drive's light. It does not, however, appear in the Finder, nor Disk Utility. When that same drive is plugged into the REAR right port, it performs correctly. When we try a second USB-C drive in the FORWARD port, it too receives power but the system cannot mount it. Using different cables does not a difference.

But… when we plug one drive in the REAR port and the other in the FORWARD port, both function normally. Huh? Isn't that weird? So is it a controller issue, a short, something on the OS side?

If we try a combination of the FORWARD right port and either of the LEFT ports, the FORWARD right drive remains unmounted.

Have reset SMC, rebooted, experimented with a Guest account, yet results remain as above.

Any thoughts, please?

Thanks