Working on the vain hope that Blizzard do actually read these posts … it might be worth listing (perhaps with a wowhead link?) a few specific examples? Then they could check their system for those items and work out what happened. It might be that certain BoE items had been converted to BoP deliberately, or by accident (Blizzard have done both of these things in the past with previous items).
I’m assuming that not ALL BoE everywhere has been converted to BoP or there would be more posts about it, so it could just be that (a) you’re more observant than most and/or (b) you’re unluckier than most
*Edit - Actually I’ve just checked out our guild vault. There is very little in there now due to Blizzard stealing everything a while back … I have one item in there, https://www.wowhead.com/item=224639/expeditionary-chopper which … when you search for it on wowhead BEFORE going to the page, shows it as BoE … but when you click on the link for it, says it is BoP …
I then removed it from the Guild vault and placed in my bags … and it changed to a different item.
Started off being Expeditionary Chopper, ilevel 545, 1518-3154 damage, 790 Agility, 3942 Stamina.
In my bags, it changes to Expeditionary Chopper of the Harmonious, ilevel 561, Explorer 2/8, BoE, 1762-3661 damage, 917 Agility, 4239 Stamina, 292 Versatility, 389 Mastery.
Placed it back in the vault and it changed back again. So yes … Blizzard visual bug. Unfortunately it also means that, as with pets, you have very little way to know exactly WHAT the item is, until you take it out. It seems to retain the armor/weapon type, but mine at least increased in ilevel and stats. It’s like it reverts to it’s “base” item when placed in the vault.