When Master is being auto vacuumed, Is slave gettting exclusive lock on its table to apply WAL?
If autovacuum finds a bunch of empty pages at the end of the table, it will try to truncate those pages away to return the space to the OS.  It will need to obtain an Access Exclusive lock to do this, and that lock is recorded in WAL and is replayed by and held by the replica server for the duration of the truncation (and replay of the truncation WAL).  hot_standby_feedback will not directly prevent this lock from being taken, although it may delay the pages from becoming empty (as the tuples won't be removed if they are still needed on the standby) but sooner or later they will need to go. 

If you are truncating away a large amount of empty pages, this can take a while (but that performance was improved substantially in v10).  If the truncation on the master detects that it is blocking someone, it will stop the truncation before completion, leaving the rest of the empty pages to be truncated by some future vacuum.  But it cannot detect that it is blocking queries on the replica, so it cannot abort the truncation.  Instead of killing the autovac worker, you could have just opened a session on the master and taken a dummy lock on the table to force the autovac worker to yield gracefully.  The dummy lock does not need to be held for a prolonged period, it can be dropped as soon as it is acquired, and it can be a weak lock, even just Access Share.

As far as I know, this is the only situation in which autovacuum will take an Access Exclusive lock.