What happens to the investors at the I-829 stage after their project goes bankrupt?

If the requisite jobs were created and maintained and investment capital properly made, the I-829 should not be affected.

Yes. In June 2017, USCIS issued this policy alert and revised its policy manual to clarify the required sustainment period of the EB-5 and what redeployment is allowable, because Chinese investors were waiting so long for green cards that the project had used the funds to create the required jobs years before the investor could even enter the US. (https://www.uscis.gov/sites/default/files/USCIS/Outreach/20170614-EB5JobsAndCapitalAtRisk.pdf) So as of June 2017, the USCIS Policy Manual (Chapter 5.A.2, I-829, Evidence of Investment and Sustainment) makes clear that the sustainment period is the two years of conditional permanent resident status. USCIS reviews the evidence to ensure the sustainment of the investment for two years from the date the investor obtained conditional permanent residence. An investor does not need to maintain his or her investment beyond the sustainment period. This is so no matter what the reason the investment is no longer sustained. Robert Divine explains this and more in this helpful memo: https://www.bakerdonelson.com/webfiles/Bios/USCIS-Finalizes-EB-5-Sustainment-and-RC-Termination-Changes.pdf

Unfortunately, the issue of whether an EB-5 investor can keep its status depends on several factors, such as if the demise of the regional center in question is as a result of fraud, whether the investment can be salvaged, if the bankruptcy is either Chapter 11 (reorganization) or Chapter 7 in which the entire company has no asset that is salvageable and simply going out of business, etc. Any combination of these factors may affect the options available to investors and whether they can keep their status is on an individual basis. Advisably, consult an EB-5 attorney if it is likely to happen to you.

The status does not end automatically. USCIS will review the I-829 and will determine if the investor sustained the EB-5 investment and if the required number of qualifying jobs were created. If USCIS has a question about the case or it believes additional evidence is required, USCIS will issue a Request for Evidence ("RFE") or a Notice of Intent to Deny ("NOID"), giving the petitioner a time frame in which to respond and identifying the evidence/information needed for the response. After reviewing the response, USCIS could approve or deny the I-829. If USCIS denies the I-829 petition, it will terminate the conditional lawful permanent resident status and likely put the investor and any dependents into removal proceedings. In removal proceedings, the investor can ask the immigration judge to review the I-829 and can ask for any and all additional relief from removal.

With the I-829 already pending, it may be OK, assuming the jobs were created during the required two-year period.

If the jobs were created, the case can still be approved.