Can you confirm that the data was actually re-uploaded?
More likely, restic could not find a suitable parent snapshot because the hostname and/or backup path set changed. The parent snapshot is used to compare file metadata so that unchanged files can be quickly skipped instead of needing to be read and hashed.
The most likely outcome of what you have described is that restic read each and every file (so it took much longer) but the deduplication mechanism would still have uploaded only new data.
In the future, you can tell restic to use a specific snapshot as the parent using the --parent flag. Note that if the backup paths changed, this might not help at all as metadata is compared between files at the same path.