This issue reproduce to me on a ODA system running 18.5.
I had found out, that, despite the +DATA disk group did have plenty of disk space, tiny space allocation raise the the error ORA-15401.
After some investigation, I found that the disks were unbalanced, with a few having less than a few GB free space.
Trying to rebalance the disk group did not work
alter diskgroup DATA check all repair; alter diskgroup DATA rebalance power 11;
I tried to remove, reformat, and add again the few disks that were showing low space:
alter diskgroup data drop disk 'HDD_E0_S00_371080360p1' force;
dd if=/dev/zero of=/dev/mapper/HDD_E0_S00_371080360P1 bs=8192 count=1000
alter diskgroup DATA add failgroup HDD_E0_S00_371080360P1 disk '/dev/mapper/HDD_E0_S00_371080360p1' name HDD_E0_S00_371080360P1 rebalance power 11;
This barely helped.
I think that the issue is what is described in the bug 26492866 – UNEVEN EXTENT ALLOCATION AFTER DROP/ADD DISK WITH REBALANCE USING ACFS.
Since that fix is not available for ODA running 18.5, the workaround, to avoid space allocation error, is to recreate ACFS disks as much as possible into the +RECO disk groups instead of +DATA.