I believe this is down to restic’s design - a key part of how it finds a parent snapshot is by using the paths that were passed to backup (Backing up — restic 0.16.3 documentation). The relevant quote from the docs is: “By default restic groups snapshots by hostname and backup paths”.
You are adding another path to backup (c:\tmp2), which means restic cannot find a parent automatically, hence the “no parent snapshot found” line.
Manually specifying a parent snapshot should help with the backup speed, as discussed here: (Backing up multiple paths at once).
The docs mention you can set grouping by tags, host, and not paths, which might work to enable the parent to be automatically detected. I think it’s worth trying (maybe while using --dry-run) to see what the effect of grouping by host is, and if the parent snapshot is detected properly.