I was aware for that bug since some time so its time to push fixes into server
What was happening: When you join, the server needs a moment to load your saved data (level, money, classes). If you're registered, that only happens after you log in. Until it finishes, the server gives you a temporary class: a random zombie class, and Spetsnaz as your human class.
If you spawned or got infected before your data finished loading, you played that life with the temporary class. Your saved class only showed up on your next spawn or infection. That's why it didn't happen every time: it depended on whether you spawned before or after your data loaded.
The fix: Once your data loads, the server now switches you to your saved class straight away, even mid-round:
Zombie: you switch to your saved class on the spot (model, claws, speed, gravity and skill). Your HP is adjusted to match the new class, so you won't get a free heal from the switch.
Human: your saved character model is applied straight away instead of waiting for your next spawn.
The fix is live on the server now after a mapchange