sorry I'm a bit late !
Yes the errors did always happened on the same frames for the exact same model parts across each test iterations, but I've managed to solve the issue.
So basically, I took a glance at the number of position, rotation and scaling keys, and I had noticed that some keys were missing.
For instance, the idle animation must have 48 keys, ...
I think I've found something, the idle animation got some really really slight offsets of movements, I've printed the matrices being used on frame 1 and for all the parts not appearing on screen the matrices got the following pattern:
(0 0 0 a)
(0 0 0 b)
(0 0 0 c)
(0 0 0 1)
Those are glm matrices (column major).
Maybe due to the really slight difference ...
I import a 20MB fbx file to update some animation of a man model, then I export it as fbx but I got a 1GB huge file. The cause is the name of the imported fbx. It got non-English words in it so the 3ds Max were working incorrectly with incompatible words. So if you are living or working in a place where English is not the native language, you should check ...