The bug
When a player in Creative mode enters and leaves lava, the player does not enter and leave the fire state immediately. This is not the case when the player enters and leaves Fire or Soul Fire.
How to reproduce
Switch to Creative mode, and then place down Fire, Soul Fire, and lava in an adequate area.
Add server-side tick lag using methods such as a performance-expensive data pack for easier reproduction.
Enter and leave the lava that has been placed.
→ ❌ The player does not enter and leave the fire state immediately.Enter and leave Fire or Soul Fire.
→ ✔ The player enters and leaves the same state immediately even with server-side tick lag.
I'm unable to reproduce without tick lag. Based on the fact that you note that this only happens with tick lag, I'm going to close this as invalid.
This is a symptom of tick lag. The essence of tick lag is that server actions (including pitting out fire on entities) are delayed.