AB4-6 rivets losing their buttons

While attaching flaperon’s pushrods I found that all of the AB4-6 rivets used in the SERVICE BULLETIN 14-12-06 lost their buttons.

The rivets used to  attach F-1206J Angles to the F-1206A Bulkhead.

All 6 buttons were recovered from the bottom of the fuselage.

Not sure if this is normal and whether the rivets need to be replaced with a bit longer and possibly stronger rivets

Update:

Took another photo from different angle to demonstrate that the rivets’ shop head has been formed OK and the rivets used are as specified in the instructions (any other rivet would be shorter and would not form the reasonably sized shop head)

Update:

The rivets have been removed and replaced with appropriate size CherryMax rivets (stronger )