I just spent some time creating about 20 different scenarios to reproduce this issue with 818 (I had the issue with 816, but no longer have it set up).
I could *not* reproduce the issue with 818.
I definitely am configured to use the FC "Size Auto" column in all my locked tabs, and I generally only use locked tabs and tabs automatically created by locked tabs.
That means that there are multiple possibilities:
1 ) Something relevant in the FC code changed from 816 to 818.
2 ) A different compiler version was used for 816 and 818.
3 ) Different compiler settings were used for 816 and 818.
4 ) There is some sort of bug that requires sufficient accumulation to take place. In other words, FC must be running for a long time with sufficient action before the error occurs.
5 ) There is some scenario in which this bug occurs that I did not replicate.
6 ) In a rare occurrence, I didn't use a locked tab. But the default in FC would still be "Size Auto", no? I tried to get FC to somehow give me a regular "Size" column by default, but couldn't get that to happen without defining a new column layout.
7 ) It's caused by a hidden artifact of the OS. I had downloaded that file from the internet. When someone downloads certain (all?) files from the internet, Windows sets a property for that file which identifies it as "possibly unsafe, downloaded from the internet". I don't remember completely, but I think those properties may be stored in
alternate streams. I'm thinking that possibly I had 2 copies of the same file: one that was directly downloaded from internet, and one that I had edited, and then reverted the edits (making it bit-identical). The first file would have the OS-generated property attached, but the second file likely would not.
8 ) There is a bug in the underlying OS.
9 ) There is a bug in FC that causes tabs or tabs auto-generated by locked tabs to switch from "Auto Size" to "Size".
10 ) Something else, or I need to get my eyes or brain examined... again.

In all seriousness, we all make mistakes, but I'm 99.9999999999% sure of what I saw. I checked it
many times before posting, because it seemed so strange.
What language is FC written in? If you want, you can send me the function that returns the values for "Size Auto" columns. I promise to fully delete it using the Wipe function after reviewing it.