I'm unsure if it's still an issue in the year of our Lord 2026. But last year it still was.
" Your items may poof if you store too many complex items in your stash. In D2R, your stash size was increased to 10x10, up from legacy D2's 8x8, but Blizzard did not increase the maximum character file size of 8 kb to accommodate for the larger stash.
This means, if you exceed Blizzard's character file size threshold of 8 kb, most commonly achieved by storing lots of rare and crafted 1-slot items, items will then poof on your character until it's within the 8 kb limit."
https://forums.d2jsp.org/topic.php?t=105979610&f=148