Support replied that “your bank sent us “do not honour” messages”, suggesting that bank is blocking transactions trough Curve or funds were not available.
For me it makes not much sense as there were funds available, also same card was working fine thru Curve later.
Maybe it was local glitch as there was 1 local bank which had maintenance exactly that day and they announced that there could be interruptions to service. I can only speculate if shop where I tried to make purchases is using that bank. My underlaying card was not from that bank.
Anyway, I think all declines should be seen in Curve app.