Right click game in steam library, choose "Properties" and remove the check mark next to 'Cloud Sync.' Be sure launch settings are set to 'Always Ask' so that you can use the other options when hitting the 'Play' button in steam. Then go to the 'Installed Files" section and do the file verification... wait for it to complete and then close it out. Test game again.

My suspicion is that the 'Cloud sync' option maybe causing a little corruption when transferring from Barn back to shop. And, it could do this whenever there are loading screens... if Steam is trying to Sync your game save at the time. Cloud save seems to be problematic with this game--lots of people have had issues with it, to include myself. I'm inclined to believe it's problematic with any game that is continually auto-saving. So, I turn it off completely. But, that's just me. I do manual backups.

Thanks for the guidance,

I did the file verification, but the items are still missing. Might be an omen for me, if I will ever pursue the path to become a mechanic to not hoard spares and just buy them on demand :D

Oh yeah... that's another issue with this game version... hoarding. It causes buggy behavior. Basically, you do NOT want to be carrying too many parts on your personal inventory. Put extras away in warehouse--but not the first slot / room that you load into by default--or, sell off excess inventory. When carrying a lot of inventory, buggy stuff happens--like disappearing items and poor game performance. The warehouse also has an issue similar to that--parts randomly duplicate and later disappear--if you put too much into the first room. Use the drop down list for the extra rooms (you can rename them) and sort your items into those additional rooms instead. For example, I only keep barn maps, extra license plates, seats, and steering wheels in the first (default) room. Rest is sorted by category into the others. I don't keep a lot of those items though so, it doesn't add up to much in the first room. Just pick things you won't keep very often.

Any advice about max item, in inventory?

I just have the lost game issues.. my 32 LV account suddenly can not hit the continue...
When I check the saved file manually.. it's only contain 2 file *.txt. so I must restarting from 0 again.

I noticed that my 32 lv's account takes more time to loading warehouse, and when searching item In the inventory acreen.. at last time I played before it lost.. I dunno wether it's caused the problem or something else.

So, the original save you had may have been corrupted. That means you'd have to start over creating a new profile / save... unless your backup save was still good. In game's main menu, you can select 'Profile" and then click on your profile name. An option to load a backup save should show up there. You can try loading it. But, usually that one gets messed up too if you had managed to load into the corrupted save and then saved again... after a couple saves, the backup save get's overwritten with the current save data.

As for how much you can keep in inventory...
There's not a definite 'known' answer for this. What I do is think of the car I am going to work on next and what I will need for it. If there's anything in my inventory that will not go on that car, I store it in Warehouse or sell it off. I only carry stuff I'll need for working on that car.

Thanks for the answer.. fyi my backup save in lost profile shown ~ instead of date.. yeah maybe the save is corrupted somehow.. now I develop habit to regularly create zip file to backup the save.. after few minutes play it and create again after close the game 😁

Fortunately, the gameplay is so amazing for me, and restarting again is still worth to play..