It seems that when trying to add an item to a piece of clothing via the combine option, it sometimes doesn't work if there isn't enough space for it to be added both vertically and horizontally.

For example, pliers are a 3 slot item - sometimes you cannot add them to a piece of clothing via combine IF that piece of clothing doesn't have both 3 empty slots vertically and 3 empty slots horizontally, however you can still add them by dragging them directly into the slots itself.

What this means is that even if the clothing has 3 empty slots vertically, but has 2 or less slots horizontally, (and vice versa) then you cannot add the item vertically even though there's enough slots for it to fit.

The weird thing about this is that sometimes after relogging the issue will no longer occur on that piece of clothing, however when you try again later with a different piece of clothing is can happen again.

It also seems to affect certain items but not others.
For example
In the video below I start off trying to add the Electrical Repair Kit to the Suit Pants, and even though the Suit Pants have 10 slots available horizontally and the ERK is a 6 slot item, it will not let me combine the item when place horizontally.

Also want to mention I'm aware that I'm trying to add the items both horizontally and vertically even in situations where the item wouldn't be able to fit under normal circumstances, it was just for demonstration to show that it doesn't work either way.

What's weird is that after it doesn't let me add the ERK to the Suit Pants, I try again but this time with Rags and it works perfectly fine - the Suit Pants have 3 available vertical slots but only 2 horizontally, yet when I try to add the Rags in the horizontal position, they automatically turn vertical and are added to the pants. Wow! The technology is here!

After that I setup the inventory so that there's only 2 slots available vertically and 5 horizontally and then try to combine the 3 slot pliers in the horizontal position and it doesn't work, then I move the Apple so we now have 4 slots available vertically - in this position we have at least 3 slots available in both the vertical and horizontal position, meaning that the combine option works both ways as intended.

Now I rotate the position of the Revolver and add a Bandage to have a different inventory layout and try again, now we have 3 different rows with 3 horizontal slots open but the Pliers will not combine with the Suit Pants.

At this point I log out and log back in.

After reconnecting I try to add a Tripwire (4 slot item) to the Suit Pants with 4 slots available in both the vertical and horizontal position, the item is able to be combined from both positions as expected.

After that I try the Tripwire once again, but this time with only 3 slots available horizontally and 4 vertically, under these conditions I was expecting that the item wouldn't be able to be combined, but to my surprise it worked as intended - when trying to combine horizontally the highlight was orange and once added the item automatically rotated into position just like the Rags.

Now I try again with the Pliers, which previously didn't work at all - this time it works perfectly when combing from both positions?! okay..

I move on and try it again with a different piece of clothing, this time a Collar Dress, once again combining the Pliers worked as intended both ways.

Around 5 minutes later and 500m down the hill, I find some Patrol Pants in a military tent and am in the process of swapping my gear over, when I try to combine my final item, the Tripwire, I have 6 slots available horizontally and only 2 vertically, when trying to add the Tripwire to the Patrol Pants, it will not work.

I then move some items around so that I have 4 slots available both vertically and horizontally and the Tripwire is now able to be combined as expected.
I then move some items around once again so I now have 4 horizontal slots, 3 vertical and it no longer lets me add the Tripwire.

I test it again with a Hoodie and it works as intended.

Finally I test it with a Red Drybag Backpack, the first test I have a maximum of 3 vertical slots available and 7 horizontal slots and try to add a Tripwire, it cannot be combined in either rotation.

I moved a few items around and then relogged to see if anything would change.
I try again with the Tripwire, this time with 3 vertical and 9 horizontal slots available and once again I cannot add the Tripwire.

I relog one last time but keep every thing in the Drybag in the same position instead.
Same thing occurs I cannot add the Tripwire via combine option.