Generally that drastic of a drop is not normal, but when I briefly checked the ratio between engagements and spend day over day I do not see any unusual gaps. For campaigns using CPI charging the billing could change drastically based upon conversion attribution changing (with tracking partners providing up to date information about attribution). I would recommend to keep an eye on it and if it continues to be a worry, file a ticket from ads.twitter.com which would be triaged by the team supporting campaign performance etc.
Thanks,
John