Wind graphs 'not working'

[AndyO2]

Struggling with this one.
Wind graphs (select-a-chart), recent charts are basically showing a flat line. Wind gust speeds chart fine. All other sensors seem to chart as expected.
Live wind speeds are updating as expected, both on the CumulusMX site & on external web server (within the realtime limits set). External web server graphs are the same as on the CumulusMX instance.
Restarting the CumulusMX service, forcing it to re-sync logs from the weather station seem to have kicked the chart into life for the period it was re-syncing, the chart then flat lines again. Restarting the server (it's Linux, not Windows but still worth a try) does not help (but gives a different flat line value). Suspect this is since a change, but likely some time ago.
The image shows flat line, till about 15:16 hrs today - the CumulusMX service was re-started & resync'd - until about 18:52 when sync completed. The leg up at the end corresponds to a server re-boot (which did not trigger a re-sync).
Instrument weather station. Debian Bullseye running as a VM with serial pass through from the host. Fully patched & up to date, including Mono. No obvious issues in the logs (under MXdiags). Have been fiddling around with file permissions to try to get php uploads working (fails due to a certificate issue within Mono, but appears unrelated. Using FTP uploads (but the issue is occurring before anything is uploaded to the website, as the graphs within the running CumulusMX instance are impacted..)

I've been through settings and can't see anything weird, & looking at the files is not making much sense... thoughts welcome please!?