Hi Callum,
For use with osmo-gsm-tester we keep our own downstream fork at sysmocom: https://gitea.sysmocom.de/sysmocom/ofono
This repo contains an "osmo-gsm-tester" branch based on upstream's master (a bit outdated now, may need some rebasing work): https://gitea.sysmocom.de/sysmocom/ofono/src/branch/osmo-gsm-tester
That branch contains mostly QMI fixes and improvements (like initial call support) which hasn't yet been submitted/merged in upstream.
On top of those fixes and improvements there are a few patches named "NOT-FOR-MERGE" or "NOT-FOR-UPSTREAM", which contain some handy modifications for our own osmo-gsm-tester, which are not of interest or cannot be merged upstream. The "NOT_FOR_MERGE: decode qmi messages with libqmi" patch will help you in debugging whatever ofono is doing in QMI with the modem.
The patch "NOT-FOR-MERGE: ignore disconnecting network interface" is the patch you are looking for.
Note that ofono versions others than the branch I presented are not known to work properly with osmo-gsm-tester.
Regards, Pau