Problem in UNIX Sort Script [URGENT]

HI Experts,
I have a "wierd" question on the sorting of a file and saving the content in another.
Say I have the source file source.txt with a content such as below
S22555            D25833967         25833967    
O22555            D25833967         0644413200  
IBBUY_8564030O3   D25833909         0644370500
T22555            D25833967         0644413200  
PBBUY_8564030O3   D25833909         0644370500
P22555            D25833967         0644413200  
PBBUY_8564030O3   D25833909         0644370500
I22555            D25833967         0644413200  
When I sort on the numerical values in the text using sort -0 destfile.txt +19 -28 sourcefile.txt i am getting the answer something like this
TBBUY_8564030O3   D25833909         0644370500
TBBUY_8564030O3   D25833909         0644370500
TBBUY_8564030O3   D25833909         0644370500
TBBUY_8564030O3   D25833909         0644370500
TBBUY_8564030O3   D25833909         0644370500
TBBUY_8564030O3   D25833909         0644370500
TBBUY_8564030O3   D25833909         0644370500
                                               
meaning all T's are coming as a bunch all s's are coming as a bunch and all the other I's and P's. But all I want is something like below

SBBUY_8564030O3   D25833909         25833909
OBBUY_8564030O3   D25833909         0644370500
TBBUY_8564030O3   D25833909         0644370500
PBBUY_8564030O3   D25833909         0644370500
IBBUY_8564030O3   D25833909         0644370500

S22555            D25833967         25833967    
O22555            D25833967         0644413200  
T22555            D25833967         0644413200  
P22555            D25833967         0644413200  
I22555            D25833967         0644413200  

I want all the fileds to be sorted on the positions 19 to 28 and then put in the order sotpi.
Can you please help me getting this?

Thanks in advance,
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