MTU test reports wrong result

We've been testing the MTU test type.

Where the remote system is 1500 and the local system 9000, 9000 is reported.
The correct 1500 is reported when the test is initiated from the 1500 system at Sheffield. Is the test simply reporting the local MTU?

Example: from ps-slough-bw.perf.ja.net (9000) to lcgperf.shef.ac.uk (1500), over IPv4 then IPv6, then on the reverse (which works).

$ pscheduler task mtu --dest lcgperf.shef.ac.uk -i 4
Submitting task...
Task URL:
https://ps-slough-bw.perf.ja.net/pscheduler/tasks/8abe6168-0201-4b4e-9ff9-b296f33ef791
Running with tool 'fwmtu'
Fetching first run...

Next scheduled run:
https://ps-slough-bw.perf.ja.net/pscheduler/tasks/8abe6168-0201-4b4e-9ff9-b296f33ef791/runs/9188d3ec-bda1-4cad-973e-2b40da6f0c98
Starts 2025-01-17T14:37:36+00:00 (~3 seconds)
Ends 2025-01-17T14:37:41+00:00 (~4 seconds)
Waiting for result...

MTU ... 9000

No further runs scheduled.

$ pscheduler task mtu --dest lcgperf.shef.ac.uk -i 6
Submitting task...
Task URL:
https://ps-slough-bw.perf.ja.net/pscheduler/tasks/5e5a5246-8a70-4b95-b7f7-bc1d0087da69
Running with tool 'fwmtu'
Fetching first run...

Next scheduled run:
https://ps-slough-bw.perf.ja.net/pscheduler/tasks/5e5a5246-8a70-4b95-b7f7-bc1d0087da69/runs/aed16bfa-6cda-4f8f-b63b-a7ab9f8ed0af
Starts 2025-01-17T14:37:52+00:00 (~3 seconds)
Ends 2025-01-17T14:37:57+00:00 (~4 seconds)
Waiting for result...

MTU ... 9000

No further runs scheduled.

$ pscheduler task mtu --source lcgperf.shef.ac.uk --dest ps-slough-bw.perf.ja.net
Submitting task...
Task URL:
https://lcgperf.shef.ac.uk/pscheduler/tasks/e0367158-d7bd-4659-a816-865db5bdb998
Running with tool 'fwmtu'
Fetching first run...

Next scheduled run:
https://lcgperf.shef.ac.uk/pscheduler/tasks/e0367158-d7bd-4659-a816-865db5bdb998/runs/7ece14e3-5f19-4002-a490-dca91a659e66
Starts 2025-01-17T14:39:09+00:00 (~3 seconds)
Ends 2025-01-17T14:39:14+00:00 (~4 seconds)
Waiting for result...

MTU ... 1500

No further runs scheduled.

Reproduce the issue with the shown pscheduler task mtu commands and the fwmtu tool, comparing both directions over IPv4 and IPv6. Investigate why the forward test reports the local 9000-byte MTU instead of the path's 1500-byte MTU; done means the affected direction reports 1500 consistently.