Sorry I didn't list it out explicitly in order
Your above sequence is correct, except that after step 4, I close the shop and after I recall back to base I re-open the shop via clicking and the item is automatically bought. Further, my OPEN SHOP key is bound to middle click on my mouse, and this has also happened while I'm sitting in base.
When in base, I've noticed it happens after I close the shop and highlighted an item to be bought. In this case it typically goes: Open shop, buy items (e.g blah blah + # of pots), and then re-open shop and the game will automatically re-buy and extra item.
I've also experienced a couple more instances while in-game today. I was teleporting back, did not open the shop via binding or clicking the actual shop, and once I re-opened the shop via clicking the shop, I randomly bought a rejuvenation bead even though I never highlighted the item prior to opening the shop. This has also happened while recalling and opening the shop by clicking the actual shop.
I would also like to add that this automatic buy bug can also complete recipe items (e.g I highlight last whisper with shop UI up, close UI, and open once again upon return to base).