Explain the reason of issue and suggest the solution

Hello Nice to meet you
I am looking for someone who can explain me that the reason of issue and explain how to fix it

I am using Wordpress website
for example)

[ProblemA]

User ID : sky (original previous ID)
User ID : sky 243558 (changed ID, automatically added the number)

All the user's ID was changed like adding some number's after the original ID
Because of this issue, user don't know which ID was changed ID
so they can't login in


1. Do you know why this is happened?
I guess that the code was changed that's why this is happened

2. Do you think this will be fixed easily?


[Problem B]

Google login issue
It was working fine but sometimes it doesn't work
This time it shows "Authentification failed"

3. Why this is happened? Could you explain me detail?

4. It happened sometimes over and over again
Will it be possible to keep having this same Google login issue after fixing it?


Thank you so much
If you can give me answers with detail, I would love to pay for it and If I have a question, I will keep working with you