Hi Aisha, see, your 50% problem is solved here. It is not only the organizational culture or open forum that is a problem; otherwise, employees from other departments would have absconded too.
With due respect (please note this is my perception), I've observed that Developers (supposed to be the most logical) are the least logical and practical. For them, policies and procedures are only those that they create, not those which the company establishes.
Addressing Developer Concerns
Now, let's address your point: Developers and designers are more concerned about technology and upgrading their skills with new technology. Is your company working on an older platform? If yes, you cannot help as this will be a management decision. If not, then a team of 20 developers and designers needs to be organized in terms of a proper and equal opportunity platform. What grabs my attention is that only 8 employees out of the total strength are CONFIRMED! Surely, the new hire is unable to visualize their GROWTH. Another aspect to consider is that (I am certain) these absconding employees are probationary employees. Frankly, no company issues experience certificates to probationary employees, even if they complete the notice period, and this fact is known to your employees, which is why they are not serving it. Additionally, a 2-month notice period for probationary employees is unacceptable. You should reduce it to 15 days. For confirmed employees, a 2-month notice period is appropriate.
Improving Employee Retention
Firstly, reduce your probation period to 3 months. Secondly, examine the type of bonding these employees have within their department. Is it possible that there are a few handpicked employees who are tarnishing the culture and speaking negatively about your company in front of other colleagues? Lastly, provide a growth path to every new hire (succession planning) after the probation period. I am confident you will notice a difference.
Thank you.