Using SQL Server and Powerbuilder 2022 R2 build 2819, I found another bug:

If I have the following:

decimal ldec_value
long ll_id

ll_id = 1

ldec_value = 7.99

UPDATE my_table
     SET my_old_price = CASE WHEN :ldec_value <> my_old_price then :ldec_value else my_old_price END 
     WHERE  my_id = :ll_id;

..... You will get a SQL Server error: Conversion failed when converting the varchar value '7.990                                                                           ' to data type int. 

WORKAROUND:

Hard cast the decimal values to numeric:

 UPDATE my_table
     SET my_old_price = CONVERT(numeric, (CASE WHEN CONVERT(numeric :ldec_value) <> my_old_price then CONVERT(numeric, :ldec_value) else my_old_price END)) 
     WHERE  my_id = :ll_id;

... I shouldn't have to encapsulate the datatypes. Powerbuilder should know that a decimal value is a numeric, not a character string!