In AbstractTransactSQLDialect constructor, tinyint is defined as:

registerColumnType( Types.BIT, "tinyint" );

Which results in tinyint values being mapped as Byte values. Considering that both in MSSQL and Sybase tinyint values range from 0-255 this ends up treating values larger than 127 as negative.

Instead it should be:

registerColumnType( Types.TINYINT, "tinyint" );