When InnoDB creates an .ibd file, it first writes a dummy page 0 with no information about encryption. This may prevent Mariabackup 10.2.20 or 10.2.21 from copying encrypted data files that were craeted shortly before backup started.
Maybe we should create a 0-byte .ibd file first, and then let page flushing write the first pages? Alternatively, preallocate a zero-filled file and then let page flushing do its job.
A valid page 0 must be written to the created files before writing any other pages. Otherwise, crash recovery cannot work.