Under California law, an employer who violates its own written disciplinary policies is in breach of contract. Assuming that you were placed on a written PIP and you are performing under that plan in consideration for the employer's agreement to reinstate you after 90 days, assuming you perform according to the plan, then that is an enforceable contract, and the employer cannot unilaterially modify it without being in breach and liable for damages (which, presumably would include any bonus not given during day 90-115.
You could file a wage claim with the DLSE for the bonus if it is not received after 90 days.
The problem with a PIP, however, is that unless there are some meausrable performance metrics associated with the PIP, then you would have no means of proving that you performed. So, you may want to wait out the 115 days, and then assuming that you are reinstated, then file the wage claim -- otherwise, the employer could simply claim that you did not perform, and so you would have no bonus, and then probably no job.
Hope this helps.
NOTICE: My goal here is to educate others about the law. I am always available to answer your follow-up questions after you click Accept – however, if you do not click Accept, the website gets paid, and I receive nothing. This is true, even if you are on a subscription plan. So please click Accept, so that I will be able to continue to provide this service for others in the future.
And, if you need to contact me again, please put my user id on the title line of your question (“To Socrateaser”), and the system will send me an alert. Thanks!