Ah, in fact, I could only see 0.12 in the app, but when I uninstalled that, 0.13 appeared! I’d love to have a page I could watch somewhere that showed me what was definitively available - is there something somewhere?
You probably had a debug build or a self-signed release installed in your device. F-Droid apps are signed by them (except when your app is reproducible), and when there’s a signing key mismatch between what you have installed and what the repo holds, then the F-Droid client doesn’t show updates, IIRC.
There’s an option on the F-Droid client’s settings to “Show incompatible apps” (or something alike) which makes F-Droid show updates even when their keys mismatch, I guess.