I would check that Khmer flavor units made availale with AL have matching tags between ArtstyleInfo and ArtDefines.
Also, does AL trigger era change? If so, check Khmer flavor units for matching tags for middle/late eras.
Thanks for your help, the problem seems to be more elusive however.
EDIT: I have investigated this further, here is what I now know (unfortunately it still does not point to the source of the issue).
Giving the Khmer AL does not change their era, they already have some other industrial techs, like Scientific Method and Steel.
Giving them another industrial tech (e.g. Combustion) does not prevent the CTD.
All units the Khmer are currently building can be created in world builder without problems.
The Khmer do not finish researching Rifling in 1480, so they do not even get a new tech in 1480.
To the best of my knowledge they do nothing whatsoever in 1480 that they haven't been doing for the previous turns as well (building units and buildings they have built numerous times before already), yet they crash in 1480 unless they get AL (and AL only).
In any case, giving them an additional tech should at most allow them to build more units, it does not take choices away, so from my understanding it should not solve a CTD, at most it should cause one. The only way I can think of that adding a tech could prevent a CTD were if it triggered an era change which in turn triggered an art style change (early / middle / late), thereby preventing the use of a wrong definition for the prior era.
This is not the case here however, all units can be built and giving them AL does not trigger an era change in the first place.
For all I know, there is nothing in my xml which causes this crash, so there is nothing I can do in turn to fix.
Any ideas and suggestions are welcome (or a confirmation that that does happen in 3.13 without my mod as well).