TOPIC: trouble field parsing when registering an event, firstname field

Hello,
we use dtregister with community builder for a long time without any issue, now after upgrading to joomla 3.4.8 we come to this issue and have then upgrade dtregister to latest version 3.1.6

This only occurs when the users has two part in his lastname.
when registering to an event, the first part of the LASTNAME is automatically added to the firstname field.
example of a user:
firstname=frederic
lastname=schmit lanson

When this person register to an event, this is the result of the autofill function in dtregister form:
firstname=frederic schmit
lastname=lanson

Any idea where does this comes from ?

thanks for your help