An output mac address is 17 bytes

         1
12345678901234567
00:11:22:33:44:55

but in net/batman-adv/distributed-arp-table.c

int batadv_dat_cache_seq_print_text(struct seq_file *seq, void *offset)
{
[...]
			seq_printf(seq, " * %15pI4 %14pM %4i %6i:%02i\n",
				   &dat_entry->ip, dat_entry->mac_addr,
				   batadv_print_vid(dat_entry->vid),
				   last_seen_mins, last_seen_secs);

%14pM is odd as this should not emit the last byte of the
mac address.  So given the example above, it would output
00:11:22:33:44

Is that what's really desired?

If so, I'd suggest using something more obvious like %5phC