Hi folks,
I’ve been battling with this exact same issue on-and-off for quite some time now over the last several years.
Like other reports here, for me the problem manifests most obviously as DHCP problem for iPhone, but like some other posts in this thread, the problem also tends to present as other strange behaviours too, with ping packet loss, some apparently random IPs intermittently not reachable and so on. Every time I have encountered the problem, I’ve been looking for network loops from badly configured bridges, and often after tidying up some unusual bridged vlan configs, the problem seems to go away… for a while 
But today, after battling this one for more than a week (not full time, fortunately I think I finally found the culprit: one of the bridges on the broadcast domain was set as proxy-arp. I can only guess that it was set that way at one time when that device was acting as VPN server access for remote access to the connected subnet, but the particular router was not performing that task any more, so it was easy to just change back to arp-enabled - problem immediately resolved! 
Investigating further, I’ve found out that this problem seems to only happen when the proxy-arp bridge has one or more ports that are non-edge. I realize now that this is why in the past when I go playing with bridge configurations to discover that the problem mysteriously goes away, it was only because I happened to fluke on a configuration that put the offending bridge onto an edge somewhere (out of sight, out of mind!
After reading through previous reports in this thread, I can see a LOT of symptoms and apparent solutions that make a great deal of sense with this discovery in mind - there is mention of VLAN and VPN and STP and so on, which are all features that tend to be in play for this problem to happen. From me, big thanks goes to @benniehier who made a post mentioning “add arp for leases” which got me thinking why that would make any difference… and that triggered a memory snippet of seeing some interface with proxy arp enabled - bingo!
I’m posting this update because I reckon I won’t be the last to come across this problem, and so adding this outcome as a potential solution to those who follow
Cheers!
Mike.