severe data loss due to broken cleanup mechanism

Cleanup mechanism seems to be broken, at least for my setup and in version provided with precise.

Setup:
Precise with 2 accounts set to backup daily with "at least a month" to local volume.
Both account had backups for 3 last months.
Each full set take ~1GB and incremental are ~25MB.
Temp folder resides on separate volume with at least 2x more space then required for single set.
Target volume had 90% / ~50GB free.

Problem:
After last full backup, both account left with only latest full set and volumes of 1 or 2 sets before latest, but WITHOUT full-signatures and inc-signatures, making backup useless!
Cache (~/.cache/deja-dup) contained only signatures of latest set.

Expected:
Even if "low space" was triggered on 90%, which seems to be bad idea, at least 2 working recent sets should be left.

duplicity: 0.6.18-0ubuntu3
deja-dup: 22.0-0ubuntu2