This bug isn’t specific to the PTR (it’s been happening since the start of D2R) but figured it’d be good to post it here for visibility and since this PTR seems focused on bug fixes.
D2 (and D2R) has a 8 KB limit for save files (.d2s) files which includes your character info like stats, resists, gold, quest progress, etc. as well as your items, inventory, and personal stash (shared stash is handled by separate .d2i files and doesn’t count towards the 8 KB limit).
The problem with the 8 KB limit is that the personal stash size was increased in D2R to 10x10 from 6x8 in D2 LoD, meaning it’s much easier to hit the 8 KB limit when you have a lot of small items with lots of data like crafted amulets/rings, rare amulets/rings/jewels, etc.
If you surpass this 8 KB limit then items will be randomly deleted as a failsafe (if the game tries to load any files bigger than 8 KB it will crash).
Thus, the save file size limit should be increased to 10 or 12 KB to prevent this bug from occurring.