Process variable update issues

I have a list type process variable which stores comments added by user task forms. Now one process instance may have several active tasks in parallel. I am loosing some of the user comments when 2 tasks are trying to save/complete their tasks at the same time. Is this is a known issue? Are there any ways to tackle such scenarios?

You have a DB locking issues, where you are trying to have two transactions write to the same variable at the same time. This is a Anti-pattern.
A few possible options:

  1. Have you looked at using the Comments API for user task?
  2. Have each user task generate a different variable, and once all tasks are completed you merge the variables together.
  3. take a look at this for other possible scenarios and understanding of the anti-pattern you are following: Pattern Review: DMN Looping for Array Input
  4. take a look at this: https://github.com/StephenOTT/camunda-concurrency-helpers-process-engine-plugin as a another variation of solution to collect your data in parallel and merge it on save.