I am not using Yahoo. Idk. I tested it multiple times to make sure I am not making up drama. I would post screenshots but I can't be bothered to edit them for privacy right now.

The disclosure mail has this in source (from OMA perspective):

X-Icloud-Hme: p=HME@icloud.com; d=; f=REAL@icloud.com; r=to; e=OMA@OMA.COM; s=OMA@OMA.COM

I know little about mail, but I think it's pretty evident there is a fuck-up, because the HME and real mail address should never be found next to each other anywhere. I kinda suspect this is meant to be forwarded to the sent box like this, but got forwarded to OMA or something.

Again, I checked multiple times, the iOS mail client shows the HME in from-field. It could be, this is "just" a bug in the iOS mail client. I presume the OP found something generally wrong with HME. But only the abyss I see here makes me absolutely not trust Apple with this anymore.

Did you do 2. and to the same OMA? The thing is, initiated from from iOS client, the ping pong goes fine-ish (despite disclosing HME usage) first time, but then "reusing" the alias, exchange initiated from same OMA is the important differentiation, apparently. There must be some issue with header rewriting, threading, idk... I presume OMA structures the header differently and does not trigger HME response as Apple expects. Or Apple already got a HME translation route for the OMA, and can't make a new one, fails to reuse the old one. Some mail servers may cut some X-whatever meta data. I mean, "X-Mailer: iPhone Mail" is cringe as fuck bloat...