It is probably a resource error.
I use Royal's server files for the soul taming repack, I am using the 8.1, 8.3, 9.1 server file resources for the improved soul taming .lua files
I use an 8.1 client with Royals and 8.1, 8.2, 9.1 file resources combined.
I do not have the same issue with durability.
Maybe while editing a resource a column has been interchanged and messed something up.
Many of the 9.1 RDB files do not work properly when imported, maybe there has been some sort of conflict with a 9.1 RDB resource.
I am not really sure what got broken on you, but I can try to take a look and figure out what.
I inserted a stage 5 tortus for comparison and it inserts broken but it repairs to 100% durability.