Most likely it's actually the proxies themselves. I used to try creating accounts with proxies through the MP embedded browser and also through proxifier and bluestacks before but always came into problems. The latest is that proxies really should only be used for running accounts, not creating them.
Account creation is the biggest flag of all. If you use the same device details on an IP, IG will flag it, and most likely your dedicated proxy has been used to create IG accounts before. One provider of proxies I know straight up bans access to the IG creation server as that causes the biggest amount of later bans/proxy rejects.
First off, are you definitely changing every single variable with memu including the mac, android id, device model, telecom operator and number for the memu instance?
Are you checking to see if the IP is definitely being picked up and used by memu? Just go to whatsmyip.org within the browser in memu to see.
If you are definitely doing all of this and still having issues when using a completely fresh proxy, you'll probably not be able to use this method unless you use the phone method most people use. If you are using semi-dedicated (i.e. not dedicated at all) that is your issue and those proxies shouldn't really be used at all.