Retention confusion about hourly propagation to daily

I was analyzing as one of my backup HDDs started to slow down and got somewhat confused.
When I have this:
```
transaction_log /var/log/btrbk.log

snapshot_dir snapshot

preserve_day_of_week sunday
timestamp_format long
snapshot_preserve 24h 7d

volume /mnt/FS_TREE/
subvolume root
target send-receive /mnt/HDDs/backups/snapshots/desktop
target_preserve 24h 15d 4w 12m
target send-receive /mnt/BigHDD/backups/snapshots/desktop
target_preserve 48h 30d 8w *m
```
I'm noticing that hourly is propagating, for example, I have 24 hours backup of may 31, june 1, ...

Is there a way to store, last 48 hours, and so only one snapshot for each 15 days, 4 weeks, etc? It is getting somewhat insane, with 2533 snapshots in my backup HDD (for this backup, at total it is at 7k snapshots for all backups in this HDD).

Start by reviewing the retention configuration shown in the issue and checking how target_preserve combines hourly, daily, weekly, and monthly intervals. Compare the resulting snapshot set with the requested 48-hour retention and longer periodic intervals; done means the behavior is explained or the configuration issue is corrected and verified against the reported snapshot counts.

Written by the indexing model from the issue text.