The 80 is a context-specific tag 0. Please note that "AUTOMATIC TAGS" is used at the beginning of the module. This indicates that all SEQUENCE, SET and CHOICE types will have context specific tags for their components starting with [0], and incrementing by 1 for each subsequent component. This way, you don't have to worry about tag conflicts when creating your messages, especially when dealing with components which are OPTIONAL or have a DEFAULT value. If you change "AUTOMATIC" to "EXPLICIT" (which I would not recommend) you will see the [UNIVERSAL 12] that you were expecting in the encoding.
Please note that AUTOMATIC TAGS applied only to tags on components of SEQUENCE, SET, or CHOICE. It does not apply to the top level components, which is why you saw the [UNIVERSAL 16] for the SEQUENCE rather than seeing a context-specific tag there also.