SPFX solutions deployed to the site collection scoped app catalog (not the tenant app catalog) do not seem to work for external/guest users. In the case of an application customizer a blank page is displayed (effectively breaking the entire site), and in the case of a webpart the the component is not rendered on the page. The same SPFX solution works for non-external users OR if deployed to the tenant scoped app catalog. This affects modern sites but does not seem to affect modern pages on classic sites.
I don't have specifics on what was done to fix this but I did receive this from MS Support on the 7th of August: "The fix is currently rolling out to all tenants. While some tenants already have the fix deployed, [your tenant] is still not corrected yet. The full roll out can take up to 4 weeks, but we should have it applied to [your tenant] in a few days. Other tenants will have it soon as well (if they are not corrected already)."