Missing “create merge request” button in issue details page

Summary

I find this problem has been going on for a long time, and it seems that version updates have not been solved.

This problem only occurs in some projects.

all projects I am owner and I logged in.

Steps to reproduce

• go to an project and go to issues
• enter one issue
• you will see follow the “show all activity” button ---"checking branch availability....."
• The right situation:then the button "create merge request" will shown
• but in some projects after a moment the "checking branch availability....." will hide and no button show(I don't know why)

I use Gitlab 12.2.3 ce and I do not know this issue maybe Is it related to permissions or/and project repository settings?

What is the current bug behavior?

some projects do not show “create merge request” button in issue details page

What is the expected correct behavior?

should show “create merge request” button in issue details page