The issuer's authorization system didn't answer — maintenance window, outage, or timeout. The card is fine; the bank is asleep. Spikes at 2–4am local time are normal.
Retry in a few minutes. Some networks 'stand in' and approve small amounts on the issuer's behalf during outages — which is why your card sometimes works when the bank's app is down.
A soft decline: the condition is temporary. A retry — later, or with a different method — can legitimately succeed.