then I had to delete all partitions with Gparted to proceed with installation.
Maybe you could tell us what happened exactly ?!
I can remember this fix being applied :
Quoteo installer: ufs: flush the disk to avoid spurious partitioning errors
From the 26.1.1 Release Notes : https://forum.opnsense.org/index.php?topic=50704.0
And IIRC someone posted that this was only a UFS thing and ZFS did not have/never had that issue ?!