0. ALWAYS PROVIDE INFORMATION ABOUT MODLOADER (Fabric / Forge) AND MOD VERSION (For Example 1.19.2-8.2.10)
   Neoforge 21.1.146, Traveler's Backpack 1.21.1-10.1.0

1. Describe the bug
   Backpack dissapears when putting on using shift + left click

2. Write steps to reproduce the bug
   Take any backpack
   Place it on the floor
   Hold shift + click left mouse button
   Backpack should be on, but is not
   What is expected behaviour
   Backpack should not dissappear

3. I have following mods:
   balm-neoforge-1.21.1-21.0.20.jar
   cloth-config-15.0.140-neoforge.jar
   connector-2.0.0-beta.7+1.21.1-full.jar
   continuity-3.0.0+1.21.neoforge.jar
   create-1.21.1-6.0.4.jar
   forgified-fabric-api-0.107.0+2.0.26+1.21.1.jar
   iris-neoforge-1.8.8+mc1.21.1.jar
   Jade-1.21.1-NeoForge-15.10.0.jar
   jei-1.21.1-neoforge-19.21.0.247.jar
   journeymap-neoforge-1.21.1-6.0.0-beta.44.jar
   lambdynamiclights-4.1.0+1.21.1.jar
   Patchouli-1.21-88-NEOFORGE.jar
   reeses-sodium-options-neoforge-1.8.3+mc1.21.4.jar
   sodium-extra-neoforge-0.6.0+mc1.21.1.jar
   sodium-neoforge-0.6.13+mc1.21.1.jar
   sushigocrafting-1.21-0.6.4.jar
   titanium-1.21-4.0.36.jar
   travelersbackpack-neoforge-1.21.1-10.1.0.jar
   waystones-neoforge-1.21.1-21.1.5.jar
   WI-Zoom-1.6-MC1.21.1-NeoForge.jar

Note: Backpack doesn't dissappear when other backpack is already worn

You are using very outdated version of travelers backpack for 1.21.1, please use latest - in this case 10.1.19

I tried one of the newer versions, so far the issues seems resolved.