Just did some testing on my server with bots. I think I now know at least one necessary condition to cause this, possibly the only necessary condition (just crashed so didn't get to vary stuff yet) --
Before I get to the test results, I should note that my reports of not getting prop disappearance were wrong, Sunshine just has very few disappearing props. But forward spawn doors, staircases, some bits of the wall/windows in cafe were vanishing. Also, it seems you cannot fix the props with record + stop, only the player models.
Test conditions were:
Server in tournament mode with SourceMod, SOAP
Running cp_sunshine_rc1a
Several bots loaded (spawned as heavy, scout, soldier)
Playing scout with a gold botkiller Mk II scattergun
The vanishing bug occured whether ragdolls were on or not, so it's not ragdolls. In fact, it occured regardless of killing or even hitting anyone.
My testing indicated that the only two consistently present variables at the time of players disappearing were:
1) In the process of firing my scattergun
2) viewmodel_fov set to a value that causes tracers to not appear
I use viewmodel_fov 0.001, but it also occured with viewmodel_fov 360 and viewmodel_fov 0.1
I got the most consistent results with r_drawviewmodel 1 and viewmodel_fov 360. Under these conditions, players disappeared immediately upon firing a single shot. With the other FOV values or with r_drawviewmodel 0, not every shot was guaranteed to cause the disappearance; it usually took between 5-11 shots in quick succession. This occured regardless of whether I was shooting an enemy or shooting a wall.
I was not able to trigger the bug with viewmodels enabled, or with viewmodel_fov 0 (which ceased to hide tracers after one of the last several updates). Also, I've already forgotten this bit, but IIRC it did not occur with r_drawviewmodel 0 while viewmodel_fov was set to a "normal" value.
At the time of this post I have not tested the following conditions--
Using any other type of botkiller scattergun
Using a non-botkiller scattergun
Using other weapons
Using other classes
Firing into the air (I was always either shooting a bot or a wall)
Other maps
Servers without SourceMod
Servers without SOAP
I did try a combination of several of these -- same class and weapon and viewmodel settings but on Walkway, both on my server (with SourceMod) and offline. I was not able to trigger it on Walkway, though I have not tried since discovering the specific conditions (though I was operating under them anyway). This indicates it could require SOAP to be loaded -- though obviously not active, if people are having it during scrims.
While there is more testing to do, I would tentatively conclude that firing a hitscan weapon while using a viewmodel FOV value that removes tracers is likely the cause of this bug. Possibly more likely with weapons that cause more tracers (e.g. scatter/shotgun rather than pistol), and probably more likely when firing rapidly (as I was almost always able to trigger it within 11 shots rapidly, whereas it seems to take more time when actually playing). Other conditions such as SOAP are not certain. It is also worth noting that because changing the viewmodel_fov did seem to change the frequency of occurence, that it may not specifically need tracers to be hidden, that may just make it more likely.