Work Manager on Chinese ROMs like Xiaomi and Oppo, when under battery optimization, increase the scheduled delay of work by several hours., however, I have noticed some apps
Similar questions have best asked in the past here and here.
This has been also asked on WorkManager's issue tracker: are the Chinese manufacturers (Huawei, Oppo, Xiaomi...) supported?
To summarize: this is a known issue with some Android OEM that heavily modify this part of Android for battery optimization. There's nothing that WorkManager can do to solve this issue.
Aside having you application added to the whitelist, you can really only report the issue:
You can take a look at the exact instruction to add to your app in a whitelist for the different devices on the amazing website don't kill my app, also I've seen this library on github that can be useful to simplify the process for your users.