Via the official Discord:
Escape from Tarkov Official #eft-announcements
Today at 8:34 AM UTC
Attention! We prohibited stacked backpacks on your character. You still can do it in your stash, but not in your character inventory. If you have any bugs related to this change – please report it via launcher. Thanks!
Twitter post:
Here's a helpful post by /u/saviorx32s who gives
If you are experiencing bugs or issues with this, please submit a bug report via the Game Launcher.
Source: https://www.reddit.com/r/EscapefromTarkov/comments/hgq4f4/stacking_backpacks_now_prohibited_on_your/