What's new since build 18
Nothing. Build 19 is build 18, re-uploaded.
TestFlight will not hold two active releases to a group at once, and does not say so. Build 18 was activated while the previous release was still active, and deactivating that previous one afterwards left both deactivated in practice — while the console went on showing build 18 as the active release. Testers were told the test was no longer active, so the release looked healthy from our side and was dead on every phone.
A build cannot be brought back from that state, so build 18 was re-uploaded under a new number to get everyone a working release. That is the whole of what build 19 is. If you are on it, you have exactly what build 18 contained.
Build 20, whose notes cover everything since build 18 — which is everything since the last time anything actually changed for you.