Urbackup on Synology NAS: space is running out but it's not

My Urbackup server is hosted as a docker container on Synology NAS.

Based on the Urbackup GUI statistic, the backup is about 245 GB.
Based on Synology Storage Manager statistics, the Urbackup folder size is 990 GB.

With a 1TB disk, Synology shows a warning that space is running out.
As a temporary solution, I connected a 4Tb disk and migrated the Urbackup docker folder.

Based on my knowledge and this post it’s a normal situation when hosted OS can’t recognize real folder size. But in my case, this is very inconvenient.

How are you guys dealing with such kind of situation?