There will be no loss of data, because the standby server will rollback the
uncommitted transactions, just as will the primary server when it is brought
back up and goes into crash recovery.

You will loose data that has not be been committed on the primary server,
but that has nothing to do with HADR and is normal for all modern database
systems.

BTW, you might want to consider using Nearsynch instead of Synch mode. The
only difference is that in synch mode DB2 guarantees that the logs have been
sent to the standby and written to the HADR buffer and also written to disk
on the standby when a transaction is committed; whereas with Nearsynch DB2
guarantees that the logs have been written to the HADR buffer on the standby
(which eventually will get written to disk). It is inconceivable to me how
one could loose any data with nearsynch unless that standby server crashed a
few milliseconds after the primary server crashed. If that happened (both
servers crashed at the same time) you would not do a takeover to the standby
server (since it would also be down), so you could not loose any data, and
when you brought the primary and standby server back up, the logs would get
in synch.